The estuarine triplefin is primarily a marine species, but it also occurs in river estuaries throughout New Zealand. It can be easily distinguished from other freshwater species by the presence of the three dorsal fins, a feature it shares with other members of the Tripterygiidae family. Although the other members of the family are strictly marine dwelling, there are a number of poorly differentiated species and the taxonomy of this family is in confusion. Until fairly recently, this species was known by the scientific name of Tripterigion nigripenne, but it has had several other names over the years, including T. robustum and T. varium. The current consensus is that it belongs in the Grahamina genus, with the specific name still in doubt.
The estuarine triplefin is found throughout New Zealand and is probably much more common that the distribution map indicates. This is because it probably does not actually live in flowing fresh waters, only going upstream as far as the tidal influence extends. Tidal habitats are difficult to sample with conventional methods. It is also an abundant species on rocky shores along the coast. Triplefins reach maturity at about 55 mm in length, and commonly reach sizes of 80–100 mm.
Breeding in triplefins appears to occur over an extended period from winter through summer. Like members of the Gobiomorphus genus (bullies), male triplefins establish and defend territories during the breeding season, and try to attract and breed with as many females as possible. After the eggs are laid in a primitive nest, which is usually the underside of a large rock, the male remains to guard them until hatching.