[7] The New Zealand grayling was a medium-sized fish which, when matured, measured between 30–45 cm in length and slender in shape with the presence of an adipose fin.

As an amphidromous fish, during their migration, it was found that graylings, originally silver in colour, would darken to a grey or brown, and turn occasionally gold, while the underbelly remained light.

[8] The dentition and digestive system of the New Zealand grayling suggests the fish was omnivorous, feeding on both aquatic larvae such as caddisflies,[9] and plants such as waterweeds growing on the rocks of riverbeds.

[3] It was found in freshwater rivers and streams during autumn and winter, and migrated to New Zealand’s surrounding seas in early spring.

A traditional way the Māori people hunted the New Zealand grayling was by using a basket trap known as a hīnaki, lodged in a river to capture fish swimming downstream.

Deforestation has been identified as a process that threatens freshwater systems as runoff can lead to habitat degradation, causing erosion and increased river sediment.

Rivers and streams that supply an optimal habitat for ecosystems are known as sources, whereas those which do not provide the same level of environmental characteristics due to pollution, overfishing, or introduced species, are known as sinks.

The increase in hunting for the fish, as it was in abundance, led to a decline in the population and eventually extinction, as the reproductive capacity of it was inundated.

[16] The final factor believed to have triggered the rapid decline and subsequent extinction of the population of the New Zealand grayling is the effects of introducing trout species from Europe.

[18][19] The introduction of both brown and rainbow trout affected endemic species, such as the New Zealand grayling, through direct predation and competition for food and habitat.

S. parasitica is inevitably introduced with salmonids and is strongly implicated in very severe fish kills of the closely related Australian grayling soon after trout introduction in Australia.