The mountain water rat is known from high-elevation forest in western New Guinea. It has strongly reduced ears and a long tail with a white tip.
Compared with its close relative, its external ears are strongly reduced; the white terminal part covers about one-third to one-half of the tail. Its underside is dark silvery gray.
It is known from upper montane forest at 2,800–3,600 m. Both Baiyankamys species are reported to use burrows beside streams, rivers, and lakes and to feed on aquatic invertebrates and small vertebrates.
An extinction-risk assessment has not been added yet.