The flathead galaxias is another of the recently discovered non-diadromous Galaxiidae that are only found in Otago, primarily in the Taieri River catchment. It was formally described as a new species in 1996, and its scientific name refers to its distinctly flattened (depressus) head (ceps). Other non-diadromous galaxids with flat heads have been found in the Clutha catchment and in Southland, but recent comparisons by Otago University suggest these are distinct from G. depressiceps with 3 new species awaiting formal recognition. Clearly, sorting out the taxonomy of the Otago/Southland galaxiid complex is going to take more time.
At present, this species of flathead galaxias is known mainly from streams in the Taieri catchment, but also the Shag, Waikouaiti and some coastal streams south of the Taieri. The flathead galaxias is difficult to distinguish from some of the other galaxiids that appear to be restricted to the Taieri catchment, although the number of caudal fin rays can be used to distinguish flathead galaxias (16 fin rays) from the dusky (14 fin rays) and Eldons galaxias (15 fin rays). Telling flathead galaxias apart from roundhead galaxias and koaro, also with 16 caudal fin rays, is more difficult, relying on the teeth, colour pattern, and the flattened head. Flathead galaxias often also have a golden stripe down the centre of the back to the dorsal fin and a golden belly. These three species never occupy the same stream, but koaro and flatheads or roundheads and flatheads do co-exist, albeit at few sites.
The distribution of the flathead galaxias is fragmented, possibly a consequence of impacts from the introduced brown trout. The preferred habitat for this species is cobble/boulder streams in tussock grasslands, and most populations occur above large waterfalls. Flatheads are found at altitudes of 140-1130 m in Otago, and they can live in steep mountain streams. Flatheads reach a maximum size of 168 mm, but are usually less than 125 mm.