Ansonia kanak is a lowland species. According to the original description (Matsui et al. 2020) the species is diagnosed as: "A small-sized Ansonia, males 19–20 mm, females 23–24 mm; head and body flattened; tympanum visible externally; tips of outer fingers spatulate; tip of first finger not reaching disk of second when fingers are adpressed; fifth toe with two phalanges free of web in both sexes; mandibular spinose tubercles absent; nuptial pad absent in males; no interorbital ridges; tarsal ridge rarely present; no light band from eye to arm." 

Type locality is Gungung Mulu, Camp 2 site. See also ASW.

The toad is small in size, smaller than most other Bornean Ansonia, but similar in size to A. teneritas, A. platysoma, and A. minuta. It is brown to olive in ground color. The warts on the back are rather smooth. The head is flat in profile and the snout is protruding.

Adults will breed in clear rocky streams. Males call from vegetation along the stream. Other details of the reproduction are unknown.

The tadpoles are medium sized and possess a sucker-type oral disc directed towards the substratum. A belly sucker is absent. The tadpoles are similar to those of A. hanitschi but remain smaller; the largest specimen documented so far measured 22 mm. The tail is quite strong and long, but tail fins are low in the proximal half of the tail. Note the pigmentation pattern of the tadpole. Tadpole cling to rocks in the current and graze on rock overgrowth, particularly at night. Haas et al. (2009) described the tadpole as A. platysoma, however DNA evidence was congruent with that for the later described A. kanak.