The species is endemic to northern Australia, and is a wide-ranging, actively foraging, opportunistic predator of aquatic and riparian habitats.

The tail is strongly compressed laterally, with a high median dorsal keel, and is about 1.5 times the length of head and body.

[7] Mertens' water monitor is found in coastal and inland waters across much of northern Australia, from the Kimberley region of Western Australia, across the Top End of the Northern Territory and the Gulf Country, to the western side of the Cape York Peninsula in Far North Queensland.

[4][8] It's diet consists mainly of freshwater Holthuisana crabs, which are reported to make up 29-83.7% of the biomass found in gut and scat samples.

Arthropods including spiders, beetles and water bugs, while frequently eaten, make up a small proportion of ingested prey biomass.