It is a slim, narrow fish with a forked tail and a mottled, spotty pattern, typically about 10 cm (4 in) long when fully grown.

It lives in fresh water, but spawns at river mouths and spends the first six months of its life at sea, returning en masse in spring.

Adults are mainly found in still or slow-moving water in the lower parts of coastal streams and rivers, or around the edges of lagoons; they can tolerate a wide range of natural conditions.

[6] Common galaxias can become land-locked (such as in five lakes in Northland, New Zealand), feeding and breeding in large beds of reeds.

[10] Adult fish typically reach sexual maturity at one year and spawning is triggered by changes in day length and temperature.

Egg mortality occurs from excess exposure to sunlight, predation from mice and spiders, grazing and trampling by livestock, mowing of bankside vegetation in urban areas, and flooding.

This marine dispersal phase is a critical part of the common galaxias's lifecycle, because it gives larvae from different populations or rivers the opportunity to 'connect'.

[16][17] Following metamorphosis, adult spend around 6 months in fresh water, where they gain sufficient growth and energy to begin investing in reproduction.

In New Zealand, Deretrema philippae (=Limnoderetrema minutum) is known to parasitise the intestine (and possibly gall bladder) of the common galaxias.

[18] The juveniles are caught as whitebait while moving upstream and are much valued as a delicacy, leading to their protection with controlled fishing seasons to preserve adult populations.

In parts of New Zealand, this species' spawning habitat has become degraded due to activities related to agriculture, urbanisation, and land-use change.

Innovative methods to restore the riparian spawning habitat include using straw bales as a temporary replacement for vegetation.

[20] Straw bales provide the same conditions and physical structure as natural vegetation, enabling the eggs to develop successfully.