In many respects the black mudfish is similar to the brown mudfish except that it occupies the northern part of the North Island. Its distribution pattern is a good way to tell it apart from the brown and Canterbury mudfish, but the distribution of the newly discovered Northland mudfish overlaps that of the black mudfish. Black mudfish can be distinguished from the Northland mudfish by the number of caudal fin rays; the Northland mudfish has only 13 or fewer rays whereas the other mudfish usually have 14 or more.
Black mudfish live primarily in swamps and wetlands and are found from the Mokau River catchment in the south up to Kaitaia in the north. They are quite abundant in the Waikato region, particularly in Whangamarino Swamp, and also occur on the Hauraki Plains. They have a similar life history to brown mudfish, with spawning taking place at the beginning of the wet season and probably continuing through to early spring.
In addition to the threat to black mudfish from land drainage and development, these fish are also threatened by the presence of gambusia, an aggressive and prolific introduced fish that has a similar distribution pattern to black mudfish. Observations of their behaviour in tanks showed that large gambusia would readily eat mudfish fry, and that they chased and nipped juvenile mudfish. However, adult mudfish were not harassed, and even attacked the gambusia. Mudfish are able to co-exist with gambusia because they can survive in habitats that periodically dry up, which gambusia cannot do, and because they breed in winter, when gambusia numbers are low.