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Mertens' Water Monitor (Varanus mertensi)

Mertens' water monitor is a large, semi-aquatic goanna of tropical northern Australia. It is closely tied to permanent and seasonal water bodies, riparian vegetation, banks and basking structures, so clearing, bank works, water extraction, fire and construction near waterways can affect individuals, shelters, nests and movement connections.

Conservation status

Commonwealth, EPBC Act 1999: Endangered.

Queensland, Nature Conservation Act 1992: Endangered.

Western Australia, Biodiversity Conservation Act 2016: Endangered.

Northern Territory, Territory Parks and Wildlife Conservation Act 1976: Vulnerable.

Breeding season

Mertens' Water Monitors are active year round. Mating generally occurs from December to February, eggs are usually laid from April to June, and hatchlings emerge in the following wet season. Plan clearing around nests in elevated banks and embankments, bank burrows and other shelters beside water.

Reproductive mode: Egg laying.

Clutch size: 3 to 14 eggs.

Incubation: About 9 to 10 months, or approximately 182 to 316 days under reported captive conditions.

Size at hatching: Approximately 13 cm snout to vent length. Reported total length is 24 to 27 cm.

Age at maturity: Approximately 2 to 3 years, with maturity at about 32 cm snout to vent length.

Nest or egg sites: Sandy clay banks and embankments beside seasonally inundated ponds, rivers and other water bodies. One recorded nest was 1.5 m above the water line and 30 cm below the embankment surface.

Breeding frequency: Not recorded.

Confirm timing locally before works, because rainfall, temperature and the timing of the wet to dry season transition vary across the species' northern Australian range.

  • Mating: Mating generally occurs from December to February. Mating has also been observed in July, and reproductive activity may occur outside the usual cycle.
  • Gravid females: Gravid females have been recorded in January, April and June, and a late-stage gravid female was recorded in December. The available records are sparse and do not define a continuous gravid period.
  • Egg laying: Egg laying generally occurs from April to June, at the late wet to early dry season transition. Captive laying has also been recorded in March.
  • Incubation: Eggs laid mainly from April to June remain in nests for about 9 to 10 months, so nests can contain eggs through the wet season and into the following early dry season. Incubation periods of 182 to 316 days have been reported under captive conditions.
  • Hatchlings: Hatchlings generally emerge during the following wet season, with the highest planning concern from January to February and possible emergence from December to April. Hatchlings can enter the water and swim immediately.
  • Active period: Adults are active throughout the year and are most often encountered during the wet season. In the Northern Territory, encounters are more likely at mid-morning or in the late afternoon than during the heat of the day.
  • Highest clearing risk: The highest risk is from December to June, when clearing may affect mating animals, gravid females, nests containing eggs and newly emerged hatchlings. Maintain year round caution because adults remain active and breeding outside the usual cycle has been recorded.

Identification

This is a large, robust monitor with strong aquatic adaptations. It commonly basks beside water and usually enters the water when disturbed.

Adults can reach 1.5 m total length. The upperparts are dark olive-brown to blue-grey, with scattered yellow, cream or grey spots edged in dark colour. Spots form rings on the outer legs and at the base of the tail, becoming less distinct towards the tail tip.

The underside is bright yellow to cream-white. The lower lip is yellow to cream and may be speckled or indistinctly barred with grey-brown. The throat is unmarked, while darker markings may occur on the chest, belly, back legs, vent and underside of the tail.

The nostrils sit high on the snout. The tail is laterally compressed and has a two-keeled crest along its spine, giving it a strong swimming profile.

Total length: Up to 1.5 m

Tail proportion: About 1.2 to 1.4 times head and body length

Colour: Dark olive-brown to blue-grey above, yellow to cream-white below

Identification: Telling it from other monitors

Look for the combination of aquatic behaviour, high-set nostrils, a strongly laterally compressed and two-keeled tail, pale spotting and a yellow to cream underside.

It is often seen close to water and may remain submerged for a considerable period when foraging or escaping. A monitor seen well away from water should not be assigned to this species without checking the full colour pattern, tail shape and habitat.

  • Do not rely on size alone, because other northern monitors can be large.
  • Record the water body, bank type, basking structure and escape behaviour with photographs where possible.

Identification: Field signs

Individuals bask on rocks, mud banks, overhanging branches, dead timber, tree roots, floating vegetation, irrigation infrastructure and roads near water.

Burrows may be dug into waterway banks. Entries are usually near water level, with an upward-sloping tunnel leading to an enlarged chamber.

The species is generally silent in the supplied sources. No diagnostic call, scat description, slough description, track dimensions or eyeshine information is provided.

  • Search water margins, basking structures and bank cavities rather than relying only on tracks.
  • Inspect proposed works near banks for burrow entrances, bank cavities and basking sites before clearing or excavation.
  • Treat a monitor entering water as a useful identification behaviour, but do not chase or capture it to confirm identification.

Distribution: National range

Mertens' water monitor occurs patchily across tropical northern Australia. Its range extends from the west Kimberley across the Top End to the wet tropics of far north Queensland.

In Western Australia it occurs in the Kimberley, including the west Kimberley and east Kimberley. In the Northern Territory it occurs across the Top End and extends south to areas bounded approximately by Judbarra or Gregory National Park, Elsey National Park and the McArthur River. In Queensland it occurs from the Gulf and far north, including the area from Mount Isa to Cairns.

The distribution is bounded to the north, west and east by the northern Australian coastline and near-coastal islands. Historical records extend further south into the Northern Tanami, Ganalanga-Mindibirrina Indigenous Protected Area and Bulliwallah Station on the Belyando River south of Charters Towers.

States and territories: Western Australia, Northern Territory and Queensland

Bioregional description: Kimberley, Top End tropical river and wetland systems, Gulf country and far north Queensland wet tropics

  • Western Australia: Kimberley, including the Fitzroy region and east Kimberley.
  • Northern Territory: Top End, including river, gorge, floodplain and wetland systems.
  • Queensland: Gulf country and wet tropics from around Mount Isa to Cairns.

Distribution: Strongholds and subpopulations

Remnant populations persist in parts of the Top End, including areas associated with Kakadu, Nitmiluk, Mary River, Litchfield, Elsey, Limmen and nearby river and wetland systems. The species also persists in parts of the Kimberley and Queensland, but the supplied evidence indicates severe declines at many sites after cane toads became established.

Important island records include Vanderlin, Groote and Winchelsea Islands in the Gulf of Carpentaria; Marchinbar, Jirrgari, Raragala and Guluwuru Islands off eastern Arnhem Land; Melville and Bathurst Islands north of Darwin; and Augustus and Koolan Islands off the west Kimberley coast.

A 2006 Fitzroy catchment workshop recorded concern about the apparent disappearance of the species from the lower Fitzroy River, while noting plentiful animals in pristine upper reaches and gorges. This is local stakeholder evidence rather than a current population estimate.

Population trend: Severe and sustained reduction over the last three generations, with further decline predicted as cane toads spread.

Cane toad affected distribution: Cane toads were established across an estimated 96 percent of the species' distribution at the time of the conservation advice.

Predicted future exposure: About 99 percent of the modelled distribution is predicted to be affected, excluding quarantined offshore islands.

Population size: No national population estimate is provided.

  • All remnant populations within the cane toad affected range are treated as important for persistence and recovery.
  • Offshore island populations that remain free of cane toads are important for retaining genetic diversity.
  • Natural and artificial water bodies that connect remnant populations may be used for dispersal and recolonisation, even when the species is not currently detected there.

Habitat: Water bodies and vegetation

This monitor is strongly associated with water and usually remains within 5 to 10 m of the water edge. It uses both natural and modified water bodies where water, prey, basking sites and shelter occur together.

Recorded habitats include perennial and semi-permanent pools, springs, seeps, swamps, creeks, gorges, permanent streams, rivers, lakes, floodplain billabongs, lagoons, soaks, perennial woodland waterholes, irrigation channels and dam margins.

Riparian vegetation provides basking, shelter and foraging resources. The species can also use open banks, rocks, dead timber, roots, floating vegetation and infrastructure where these occur beside water.

Typical distance from water: Usually no more than 5 to 10 m from the water edge

Habitat climate: Wet to dry tropical northern Australia

Elevation limit: Not reported in the supplied documents

  • Retain open water, wetland margins, river banks, gorges, riparian vegetation and bank structures within and beside the work area.
  • Map connected water bodies and intervening riparian habitat, including areas that are temporarily dry or unoccupied.
  • Include artificial channels and dam margins in habitat assessments, not only natural rivers and creeks.

Habitat: Features that increase habitat value

Habitat critical to survival includes water bodies with open water, food resources, basking sites and shelter opportunities, together with connecting habitat between remnant subpopulations.

A site can remain important even when no animals are detected. Temporarily unoccupied water bodies and riparian corridors may provide future dispersal, breeding and recolonisation habitat.

  • Check for stable banks, bank cavities, burrows, basking structures, shallow foraging areas and nearby riparian cover.
  • Avoid isolating pools, channels, gorges and billabongs from one another where animals may move seasonally or during flood events.
  • Assess dry-season water retention and groundwater dependence before approving drawdown, drainage or dewatering.

Foraging habitat: Food

Mertens' water monitor is an active predator that usually feeds in water and along the water-land interface. It can remain submerged while foraging and can concentrate fish in shallow water with its body and tail.

Regular prey includes freshwater crabs, fish, insects, amphipods, spiders and frogs. It also takes shrimps, crayfish, aquatic snakes, lizards, nestling birds, small mammals, caterpillars and eggs.

The species generally takes relatively small prey for its body size. It can learn to eat carrion and cooked meat scraps around human activity, although dead fish are generally avoided.

Main feeding locations: In water and at the water-land interface

Late dry-season behaviour: May exploit contracting ephemeral pools where fish become concentrated

  • Protect aquatic prey habitat, including shallow margins, pools, submerged structure, clean water and riparian cover.
  • Prevent food waste, fish scraps and other attractants from drawing monitors into active work areas or roads.
  • Do not use bait or food to locate animals during project works.

Foraging habitat: Foraging movement

Adults move among core activity areas and may travel between separated aquatic areas seasonally. Activity area size varies with waterbody shape and type.

Radio-tracked animals moved an average of 0.7 km per day along irrigation channels and up to 1 km in a day while foraging. One animal moved 5.5 km in five days around Manton Dam.

Core activity area: 0.03 to 16.8 ha

Home range: 0.03 to 31.8 ha

Average daily movement: 0.7 km along irrigation channels

Recorded maximum daily foraging movement: Up to 1 km

  • Inspect the full connected waterway and bank margin, rather than treating a single pool or dam as an isolated site.
  • Keep fauna spotter catcher access and escape routes open towards water during clearing.
  • Record waterway crossings, culverts, drains and channel margins as potential movement routes.

Breeding habitat: Breeding biology

Breeding is linked to the wet to dry season transition, but observations outside the usual annual cycle indicate that timing can vary. Nests occur in elevated sandy or clayey banks and embankments near water.

Mating: Generally December to February, with mating also observed in July and a gravid female recorded in December.

Egg laying: Generally April to June, during the late wet to early dry season.

Clutch size: 3 to 14 eggs

Incubation: About 9 to 10 months, or approximately 200 to 300 days according to the supplied secondary source.

Hatching: Generally during the following wet season.

Hatchling size: About 13 cm snout to vent length

Maturity: Approximately 32 cm snout to vent length

Lifespan: Not reported in the supplied documents.

Breeding habitat: Nest habitat

The first recorded nest was in an artificial sandy clay embankment beside a seasonally inundated pond. It was estimated to be 150 m from a seasonally dry river stretch and 70 m from riparian vegetation.

The nest was about 1.5 m above the water line and 30 cm below the top of the embankment. Vegetation lining was reported historically but was not observed in that nest.

Recorded nest distance from seasonal river: About 150 m

Recorded nest distance from riparian vegetation: About 70 m

Recorded nest elevation: About 1.5 m above the water line

Recorded nest depth: About 30 cm below the embankment top

  • Inspect sandy or clayey banks and embankments near seasonal ponds, rivers and riparian vegetation before excavation or bank modification.
  • Retain elevated nesting banks, especially where they are close to water but above the normal water line.
  • Avoid stockpiling soil, compacting banks or changing drainage around suspected nesting areas during the egg-laying and incubation period.

Sheltering habitat: Burrows and cavities

The species uses multiple shelters beside water for overnight refuge and escape from disturbance. Shelters include bank burrows and cavities between rocks.

Burrows are dug into waterway banks, with entrances usually at water level. The tunnel slopes upwards to an enlarged chamber at the end.

Hollows between rocks and several shelters may be used by different individuals. Multiple refuges allow animals to move between overnight shelters and escape sites.

Burrow entrance: Usually at water level

Burrow form: Upward-sloping tunnel ending in an enlarged chamber

Shelter dimensions: Not reported in the supplied documents

  • Search banks at or close to water level for entrances before earthworks, trenching, bank stabilisation or vehicle access.
  • Retain rock piles, natural bank cavities, exposed roots and stable earthen banks where they occur beside water.
  • Do not block, fill or collapse a suspected burrow until it has been assessed by a suitably qualified fauna ecologist or fauna spotter catcher.

Sheltering habitat: Daily refuge pattern

Animals emerge in the morning to warm up, forage and bask during the day, then return to an overnight shelter late in the afternoon as body temperature falls.

When disturbed, they commonly enter the water and may remain submerged for a considerable period.

  • Plan pre-clearing inspections around the water margin, bank cavities, rocks, timber, roots and basking structures.
  • Keep personnel, machinery and exclusion fencing clear of water access points so animals can move away from disturbance.
  • Allow time for a submerged animal to leave an impact area without pursuing it through the water.

Threats: Development and clearing

Cane toads are the main threat, but impacts from clearing, bank disturbance, water extraction, fire, feral animals and development can compound declines in remnant populations.

Urban, industrial, agricultural and mining development can remove or degrade water bodies, riparian vegetation, banks, basking sites, shelters and connecting habitat. Subpopulations that persist near Darwin, Kununurra and Mount Isa are exposed to development pressure.

Clearing assessments must include habitat that is not currently occupied but may be used for future dispersal, breeding and recolonisation.

Typical proximity to water: Usually within 5 to 10 m of the water edge

Recorded home range: 0.03 to 31.8 ha

  • Avoid clearing water margins, riparian vegetation, stable banks, rock cavities, burrows and nesting embankments.
  • Map habitat within at least the known activity area and connected aquatic routes, rather than limiting the assessment to the construction footprint.
  • Use qualified ecologists and, where appropriate, Traditional Owners with local knowledge to assess habitat, resilience and subpopulation use.
  • Inspect before clearing and maintain a fauna response procedure for animals found in burrows, bank cavities, drains, culverts and machinery exclusion zones.

Threats: Fragmentation, roads and construction access

Fragmentation of water bodies and riparian connections can restrict movement between core activity areas and remnant subpopulations. Roads and tracks may also provide basking surfaces, but they increase access, vehicle interaction and disturbance near water.

The supplied documents do not provide a species-specific road mortality rate or a confirmed road crossing frequency.

  • Keep culverts, drains, channels and floodplain connections passable where they form aquatic movement routes.
  • Use low-speed controls and fauna awareness measures near water bodies, especially during wet-season activity.
  • Prevent vehicles from entering banks, pools, basking sites and known shelter areas.
  • Do not assume that an unoccupied connecting water body has no conservation value.

Threats: Cane toads and predators

Mertens' water monitors are highly sensitive to cane toad toxin and live and feed in wetlands that are also core cane toad habitat. Severe declines and local disappearances have followed cane toad establishment.

Freshwater crocodiles and water pythons are reported predators. Feral pigs may eat eggs and damage wetlands, while cattle and Asian water buffalo can trample nests and banks.

Cane toad impact: Severe population reductions and possible local extirpations have been recorded after establishment.

Adult annual survival estimate: About 89 percent for both sexes

  • Use cane toad hygiene and quarantine procedures when moving machinery, materials, soil or water between sites.
  • Do not create new wetland disturbance that increases access to remnant monitor populations after cane toads arrive.
  • Control feral pigs and manage stock access where nesting banks, shallow pools or riparian habitat are affected.

Threats: Fire, hydrology and pollution

Frequent severe fire can reduce woody riparian vegetation used for basking, shelter and foraging. Water drawdown can dry groundwater-dependent ecosystems and reduce the permanence of pools and wetlands.

Cattle and buffalo can increase turbidity, remove vegetation and damage earthen banks. Feral pigs can increase turbidity and prolonged anoxia, reducing access to aquatic prey.

The supplied sources do not identify a species-specific disease threat.

Water drawdown risk: Can dry groundwater-dependent ecosystems used by the species.

Fire risk: Frequent severe fire can reduce riparian vegetation and habitat quality.

  • Maintain environmental flows, groundwater recharge and dry-season water retention in occupied and connecting habitat.
  • Avoid dewatering, drainage, excessive extraction and changes to floodplain connectivity.
  • Use fire regimes that protect riparian woody vegetation and avoid severe burns along water margins.
  • Control sediment, turbid runoff, sewage, hydrocarbons and construction waste entering pools, channels and wetlands.

Survey methodology: 1. Habitat and records review

Mertens’ water monitor is a semi-aquatic lizard of tropical northern Australia. Surveys should concentrate on waterbodies and their immediate margins, particularly during the wet season when the species is more active and more often detected.

Confirm whether the project is within the species’ recorded distribution, which extends from the west Kimberley in Western Australia through the Top End of the Northern Territory to far north Queensland.

Map perennial and semi-permanent pools, springs, seeps, swamps, creeks, gorges, rivers, lakes, billabongs, lagoons, soaks, dams and irrigation channels.

Map habitat within at least 5 to 10 m of the water edge, where the species usually remains, and identify basking structures, waterway banks, rocks, branches, dead timber, roots and burrows.

Check nearby connected waterbodies because adults can move between aquatic activity areas and the species can disperse by swimming during flood events.

Survey methodology: 2. Diurnal visual and boat-based searches

Search during daylight, with particular attention to morning basking and late afternoon activity, when the species emerges, warms up, forages and returns to shelter.

Inspect water edges, rocks, mud banks, overhanging branches, dead timber, tree roots, floating vegetation, irrigation infrastructure and roads used as basking sites.

Use standardised boat-based transects where safe and suitable, because this method has been used to monitor population trends after cane toad invasion.

Record animals that enter the water when approached, as Mertens’ water monitor readily takes to water when disturbed and can remain submerged for a considerable period.

Repeat searches across suitable connected waterbodies where the project could affect movement between core activity areas.

Usual activity period: Daylight

Most frequent detections: Wet season, with detections commonly higher at mid-morning and late afternoon than during the heat of the day

Typical distance from water: Usually within 5 to 10 m of the water edge

Survey methodology: 3. Targeted camera surveillance

Deploy targeted cameras at likely basking sites, water edges, burrow entrances and access points between water and shelter where visual searches are not sufficient.

Use cameras as part of a standardised, repeatable monitoring design, because targeted cameras have been used to monitor this species at cane toad affected sites.

Place cameras so that they do not obstruct burrow entrances, damage banks or create a hazard for fauna or machinery.

Record camera location, dates deployed and retrieved, operating condition, habitat type, waterbody condition and all detections.

Survey methodology: 4. Follow-up searches and detection interpretation

Treat a failed search as inconclusive where water is turbid, access is poor, the animal may be submerged, or cane toad impacts have reduced local abundance.

Inspect banks and structures for active burrows and refuge sites, noting that burrow entrances are usually at water level and lead upwards to an enlarged chamber.

Where a monitor is detected, search connected habitat and nearby waterbodies for additional individuals rather than treating the record as an isolated occurrence.

Do not rely on a single observation to define the extent of habitat, because individuals may move between aquatic areas and some are wanderers or transients without a bounded home range.

Core activity areas: 0.03 to 16.8 ha

Home ranges: 0.03 to 31.8 ha

Average daily movement recorded along irrigation channels: 0.7 km, with movements of up to 1 km in a day while foraging

Survey methodology: Minimum effort and reporting

  • Use a documented combination of daylight habitat searches and targeted camera surveillance, with boat-based transects where the site is safely accessible by water.
  • There is no species-specific survey effort, such as a prescribed number of nights, trap nights, camera days or transect length, in the supplied documents, so justify the effort by mapped habitat extent, waterbody connectivity, access and the proposed disturbance footprint.
  • Schedule repeat searches in suitable wet-season conditions and include morning or late afternoon observations rather than surveying only during the hottest part of the day.
  • Record survey dates, start and finish times, weather, water conditions, observers, methods, effort, GPS locations, photographs, habitat features and detection results.
  • Submit confirmed records, including negative results where requested by the regulator, to the relevant state or territory wildlife database and provide records to the project regulator and land manager.
  • Report cane toad presence, habitat degradation, connected waterbodies and any suspected remnant or cane toad resilient subpopulation with the fauna survey results.

Before habitat disturbance: Avoid and minimise

Avoid impacts to waterbodies and their connected riparian habitat first. The species depends on open water, food, basking sites and shelter, and habitat may remain important even when monitors are temporarily absent after cane toad impacts.

  • Retain perennial and semi-permanent pools, springs, creeks, gorges, rivers, lakes, billabongs, lagoons, soaks, dams and irrigation channels within and adjacent to the work area.
  • Retain the water edge, banks, riparian vegetation, basking structures, dead timber, rocks, roots and existing burrows.
  • Avoid works that sever connections between waterbodies, because connected habitat supports dispersal, recolonisation and movement between activity areas.
  • Avoid changes to surface water or groundwater that could dry groundwater-dependent ecosystems or reduce the integrity of seasonal waterbodies.
  • Keep machinery, stockpiles, access tracks, spoil, sediment, concrete washout, fuels and other contaminants away from water and banks.
  • Avoid actions that remove or degrade habitat used by cane toad resilient subpopulations or habitat that may be recolonised during recovery.

Before habitat disturbance: Approvals and project controls

National status: Endangered under the EPBC Act, effective 21 December 2023

State and territory status supplied for the profile: Northern Territory Vulnerable, Queensland Endangered, Western Australia Endangered

  • Check the EPBC Act requirements before works start because Mertens’ water monitor is nationally listed as Endangered.
  • Check the applicable state or territory legislation, permits and wildlife handling requirements before survey, clearing, relocation or other disturbance.
  • Refer the proposed action under the EPBC Act if the impact assessment indicates that it may significantly affect the nationally listed species or its habitat.
  • Obtain written approval for any fauna handling, temporary holding, transport or relocation and ensure the fauna spotter catcher is authorised for the work.
  • Prepare a fauna management plan that identifies mapped habitat, survey effort, exclusion areas, stop-work triggers, handling arrangements, emergency contacts, reporting and rehabilitation.

Before habitat disturbance: Work planning and site preparation

  • Where practicable, stage works so that retained waterbodies and connected riparian habitat remain available throughout construction.
  • Plan high-risk works outside periods of high activity where practicable, noting that the species is active year round and is most often encountered during the wet season.
  • Complete the pre-clearance survey before machinery enters mapped habitat and repeat the inspection if the work footprint, access route or water conditions change.
  • Mark no-go areas, water edges, burrows, basking sites, banks, riparian vegetation and habitat corridors on plans and on the ground.
  • Install exclusion fencing only where it will not block access to water, trap a monitor between fencing and the work area, damage banks or sever movement routes.
  • Brief all workers on the species’ appearance, its habit of entering water when disturbed, the no-go areas, reporting process and stop-work triggers.

During habitat disturbance: Daily controls and machinery interface

Mertens’ water monitor can retreat into water, banks and burrows when disturbed. Work must proceed under direct fauna supervision around waterbodies, banks, basking structures and connected riparian habitat.

  • The fauna spotter catcher must inspect the active work area, access routes, water edges, banks, burrows and refuge structures before work starts each day.
  • Keep the spotter catcher in direct communication with the operator and stop machinery immediately when the spotter identifies a monitor, burrow, unstable bank, waterbody hazard or other fauna risk.
  • Maintain a slow, controlled approach near water and banks, and do not drive through pools, burrows, riparian vegetation, basking structures or marked exclusion areas.
  • Use a staged clearance sequence that allows the animal to move towards retained habitat and water rather than trapping it between machinery, fencing and the work edge.
  • Do not chase a monitor into the water, block its retreat, enter the water to capture it, or disturb a burrow to force the animal out.

During habitat disturbance: Habitat features and water quality

  • Retain rocks, mud banks, overhanging branches, dead timber, roots, floating vegetation and other structures used for basking or shelter wherever practicable.
  • Do not fill, collapse, seal or drive over burrows, which may have a water-level entrance and an upward sloping tunnel leading to an enlarged chamber.
  • Prevent sediment, fuel, concrete, rubbish, wastewater and other pollutants from entering waterbodies, because the species forages in water and at the water-land interface.
  • Stop or modify work if dewatering, diversion, drawdown, bank damage, turbidity or other activity could affect retained aquatic habitat or connectivity.
  • Do not remove riparian vegetation unnecessarily, because it provides basking, shelter and foraging resources.

During habitat disturbance: Animal found, handling and relocation

Adult size: Up to 1.5 m total length

Normal response to disturbance: Often enters water and may remain submerged for a considerable period

  • Stop work in the immediate area and keep people, vehicles and dogs away when a Mertens’ water monitor is found.
  • Allow the animal to move to nearby retained water or shelter without intervention where it can do so safely.
  • Only an appropriately authorised fauna handler may capture, handle, hold, transport or relocate the animal, and handling must follow the approval and fauna management plan.
  • Do not pick up or restrain the animal unless it is at immediate risk and the authorised handler considers intervention necessary.
  • Relocate an animal only where approved, to suitable nearby retained habitat with open water, food resources, basking sites and shelter, and avoid moving it across roads or into isolated habitat.
  • Place an injured animal in a secure, ventilated container suitable for a large monitor and arrange prompt assessment by a veterinarian or permitted wildlife carer.
  • Do not release a sick, injured or apparently intoxicated animal without veterinary or wildlife carer advice.

During habitat disturbance: Stop-work triggers and records

  • Stop work if a monitor is within the active work area, if it cannot move safely to retained habitat, or if the fauna spotter catcher cannot maintain visual control of the risk.
  • Stop work if an active burrow, occupied refuge, unexpected waterbody or previously unmapped connected habitat is exposed.
  • Stop work after a fauna injury, mortality, entrapment, pollution event, bank collapse, uncontrolled dewatering or breach of an exclusion area.
  • Record every detection, attempted capture, handling event, relocation, injury, mortality, work stoppage and corrective action with date, time, GPS location, photographs and personnel involved.
  • Notify the site environmental officer, project manager, regulator and land manager in accordance with the fauna management plan and approval conditions.

After habitat disturbance: Post-clearance inspection

Post-work checks must confirm that retained aquatic habitat remains usable and that animals have not been trapped, injured or excluded from connected habitat. Recovery may take many decades to centuries, so records and repeat monitoring are important.

  • Inspect the completed work area, retained banks, water edges, burrows, culverts, temporary fencing and access routes before demobilisation.
  • Remove temporary fencing, trenches, pipes, pits, debris and other structures that could trap a monitor or block movement, unless they are part of an approved permanent design.
  • Confirm that water continues to connect retained habitat and that no spoil, sediment, rubbish, fuel or concrete has entered the waterbody.
  • Complete a final daylight search of retained habitat and connected water edges, including likely basking structures and burrow entrances.
  • If a monitor is found, apply the approved fauna handling and stop-work procedure rather than assuming it has moved away.

After habitat disturbance: Monitoring and adaptive management

  • Repeat targeted visual searches or camera surveillance where works affected banks, water levels, riparian vegetation, burrows or movement connections.
  • Monitor habitat attributes relevant to the species, including water availability, bank condition, turbidity, riparian cover, basking structures, shelter and prey availability.
  • Compare results with the pre-work survey and investigate any decline, persistent absence, injury or change in use of retained habitat.
  • Share survey and monitoring outcomes with recovery partners, land custodians, owners and managers to support coordinated recovery.

After habitat disturbance: Reporting and offsets

  • Submit required fauna, incident, survey and rehabilitation reports to the relevant regulator and state or territory wildlife database.
  • Report confirmed records with photographs and precise locations, while protecting sensitive location information where required by the land custodian or regulator.
  • Review whether the project affected habitat critical to survival, a remnant subpopulation or connecting habitat and document the outcome in the approval report.
  • Do not rely on an offset as a substitute for avoiding impacts; where an approved offset is required, use local, like-for-like habitat restoration or creation where specified by the approval.

Rehabilitation actions: Waterbodies, banks and connectivity

Rehabilitation should restore connected aquatic habitat and the features used for basking, shelter, foraging and movement. Design should account for both occupied sites and habitat that may be recolonised as cane toad resilient animals disperse.

  • Reinstate natural or approved seasonal water connections between retained pools, creeks, billabongs, lagoons, soaks and other suitable waterbodies.
  • Restore stable, gently varied banks with access to water, shelter opportunities and suitable basking structures.
  • Re-establish riparian vegetation appropriate to the local wetland and river community, including native trees, shrubs, grasses and ground cover identified by the local ecologist or Traditional Owners.
  • Prevent excessive water drawdown and maintain surface and groundwater conditions that support permanent and seasonal aquatic habitat.
  • Design culverts, crossings and drainage structures so they do not unnecessarily block movement between waterbodies.

Rehabilitation actions: Shelter, basking and refuge features

  • Retain or reinstate rocks, mud banks, overhanging branches, dead timber, tree roots and floating vegetation used for basking and refuge.
  • Reform and stabilise suitable banks so that burrows can be maintained without being buried, sealed or exposed to machinery.
  • Do not install nest boxes as a substitute for the species’ documented aquatic shelter requirements unless a site-specific ecologist recommends them for another affected fauna species.
  • Use local materials and avoid placing logs, rocks or other structures where they obstruct water flow, trap fauna or prevent access between water and land.

Rehabilitation actions: Threat reduction and site management

  • Control weeds and prevent the spread of introduced plants through restored riparian areas and access routes.
  • Manage feral pigs, cattle and Asian water buffalo where they damage banks, increase turbidity, remove vegetation or affect nesting and feeding areas.
  • Apply a site-specific fire regime that protects riparian vegetation, shelter and foraging resources from frequent severe fire.
  • Maintain cane toad quarantine, surveillance and early intervention measures on offshore islands where the species occurs.
  • Manage rubbish, visitor access, roads and machinery so that monitors cannot become trapped and water-edge habitat is not repeatedly disturbed.

Rehabilitation actions: Success measures and monitoring

  • Monitor the restored area during the wet and dry seasons for water persistence, bank stability, riparian cover, basking structures, burrow availability, turbidity and connectivity.
  • Use repeat daylight searches, targeted cameras or standardised boat-based transects where safe to measure use by Mertens’ water monitor.
  • Set site-specific success measures before works begin, including survival of planted riparian vegetation, stable banks, functioning water connections, absence of fauna traps and documented monitor detections where the species is expected to persist or recolonise.
  • Continue monitoring long enough to detect recovery, recognising that the supplied conservation advice states that recovery may take many decades to centuries.
  • Adapt weed, feral animal, fire, water and access management if monitoring shows declining habitat condition or failure to support recolonisation.

Sources