The dusky galaxias is a new addition to the Galaxiidae family that was formally described in 1997. It has a restricted distribution in the Otago region in tributaries of the Taieri and Clutha River catchments. Although it occurs in the same general area as another of the new Otago galaxiids, Eldons galaxias, the two species have never been found together.
The dusky galaxias differs from the other galaxiids in having only 14 caudal fin rays compared to 15 for Eldons and dwarf galaxias and 16 for the others. It is generally a brown colour, but with a distinct dark and light colour pattern. It also has darker blotches behind the gill openings that are more characteristic of the diadromous banded and shortjaw kokopu. Adults are commonly up to 110 mm in length, although specimens of 150 mm have been recorded.
The dusky galaxias lives in tussock and forest streams at mid to high elevations (400-1000 m), utilizing riffle and pool habitats. Spawning occurs in spring with the fish laying their eggs under overhanging banks in riffles. These fish eat a wide variety of aquatic insects and occasionally terrestrial items that fall in the water.