This member of the non-diadromous galaxiid group has a very restricted distribution; it is confined mainly to tributaries in the lower to mid Taieri River catchment. This is another new Otago galaxiid species and it was first formally described in 1997. The name honours G.A. Eldon, who assissted Dr R.M. McDowall with his investigations into the galxiidae. Eldons galaxias closely resembles the flathead galaxias, but it has a deeper body and darker colouration, especially in large individuals. Its can also be distinguished from the flathead and other galaxiids by the number of caudal rays (15 in Eldons, 16 in the flathead, and 14 in the dusky galaxias). It shares this characteristic of 15 caudal rays with the dwarf galaxias, but their distributions do not overlap.
Although the full extent of their occurrence is not yet known, it appears that the distribution of Eldons galaxias is highly fragmented. This may be caused by competition from the introduced salmonids: brown trout, rainbow trout and brook char. The establishment of land-locked populations of native koaro in Lake Mahinerangi may have also affected their distribution.
Eldons galaxias reaches a maximum size of about 150 mm and are commonly found up to 110 mm long. Like most of the non-diadromous galaxiids, Eldons galaxias feeds on aquatic insects, occasional terrestrial items, and on rare occasions, small koura. They can often be seen during the day feeding on items drifting downstream.
Eldons galaxias tends to prefer riffle habitats, but can also be found in pools. They occupy a diverse range of streams from high altitude tussock streams to low altitude forested ones. Often they are found upstream of large waterfalls that restrict the distribution of salmonids. Spawning occurs in mid-spring, and the larvae hatch about a month or so later.