The giant kokopu normally migrates between the sea and freshwater as part of its lifecycle. It is still not known exactly where spawning takes place, but adult males have been observed migrating downstream in large numbers at the end of autumn (2). The females produce thousands of tiny eggs which upon hatching are carried out to sea (4). In the spring, the developing juveniles migrate back into the rivers in mixed-species shoals as part of the whitebait run (2) (4). Unlike some whitebait species, the giant kokopu does not migrate far inland, but instead remains in the slower-moving waters downstream, favouring areas where it can hide below overhanging vegetation and undercut banks (4). Maturity is reached after around three years, but giant kokopu may live for many years, with some individuals estimated to reach almost 30 years old (2).
An opportunistic feeder, the giant kokopu appears to take a wide range of prey items. In some parts of its range, terrestrial insects have been recorded as forming the bulk of its diet, while in other areas, aquatic prey are much more important. Landlocked adults appear to be particularly indiscriminate when it comes to food, even taking juvenile giant kokopu (2).