As its name implies, the shortjaw kokopu has an undercut jaw, with the lower jaw being shorter than the upper jaw. Koaro also have shortened lower jaws, but they are much more tube shaped than the kokopu. Shortjaw kokopu are also rather drab in colour unlike the sparkly koaro. The main distinguishing feature is a distinctive dark blotch on each side just behind the gills. Otherwise, this fish is essentially brownish with faint bands and blotches.

Shortjaw kokopu are endemic to New Zealand and do not occur on Stewart or Chatham Island. Although they penetrate well inland in many catchments, they appear to be restricted to streams with native forest vegetation. Even though it is widespread, the shortjaw kokopu is probably the rarest of the whitebait galaxiids as it is unusual to capture more than a few fish at a given site. It is usually found in streams with large boulders in pools and is difficult to catch using conventional methods. Because this fish has been so rarely encountered, little is known about its life history. In 2009, the first landlocked population was discovered in an Auckland reservoir.