The koaro is unlikely to be confused with the other diadromous whitebait species because of its shape. It is more elongate and slender shaped, almost like a tube. The sides and back are covered in a variable pattern of light patches and bands that making the koaro a very attractive fish.
Unfortunately, it is not so easy to tell the koaro apart from some of the non-diadromous galaxiids like the Canterbury galaxias and those that live in Southland and Otago. Identification problems are complicated by the fact that koaro can co-habit with the other species in the same rivers and streams. Even the experts have problems separating them, relying on technical measures such as the number of caudal fin rays to ensure correct identification.
Koaro have the ability to penetrate well inland in many river systems, and thus have a more widespread distribution than the other whitebait species. In addition to the mainland, they are also found on Chatham and Stewart Island, in Australia, and on the sub-Antarctic Auckland and Campbell Island.
Rocky, tumbling streams are the preferred habitat of koaro, and they are almost always found in streams with native bush catchments except for tributaries of upland lakes that may be above the bush-line. Studies in Australia found that koaro spawned in damp areas along the edges of the streams they lived in, relying on subsequent floods to inundate the eggs for hatching. A requirement for dampness could explain their preference for forested streams, and shows why their distribution in New Zealand has probably been curtailed by widespread forest clearance, more so than most of the other Galaxiidae.
Although koaro comprise part of the whitebait catch, they also form land-locked populations in lakes. For example, koaro populations occur in the catchments of many of the Rotorua lakes, Taupo, Rotoaira, Manapouri, Tekapo, Pukaki and Wanaka. Koaro populations in lakes were decimated by predation from introduced trout and are now much lower than in pre-European times when they provided a fishery for Maori. In lakes, where smelt have been introduced, koaro have declined even further and are either now confined to tributary streams or have become extinct.