Porphyrio melanotus, commonly known as the Australasian swamphen, is native to Australia, New Zealand, and various other islands in the southwestern Pacific. These regions are part of the Australasian realm, including countries like New Zealand, Australia, Papua New Guinea, and Indonesia, particularly on islands such as New Guinea and surrounding smaller islands.

The Australasian swamphen typically inhabits wetlands, including swamps, marshes, and the edges of lakes and rivers. These environments are characterized by shallow waters and abundant vegetation, particularly reed beds and tall grasses, which provide both food and cover for the swamphen. The climate in these regions can vary, ranging from temperate conditions in New Zealand to more tropical climates in parts of its range, such as northern Australia and Papua New Guinea.

The ecological niche of Porphyrio melanotus is primarily associated with wetland ecosystems where it plays a role as an herbivore and, to a lesser extent, a predator. It feeds on a variety of plant materials, insects, and small animals, such as frogs and snails. The swamphen is adapted to both walking on soft, waterlogged ground and swimming. Its long toes enable it to walk on surface vegetation, making it well-suited to this specific ecological niche.