The ecoregion includes the extensive swamp forests of southern and western New Guinea.

The forests lie in the lower reaches of the rivers that drain New Guinea's highlands.

Near the coast the freshwater swamp forests transition to New Guinea mangroves as the waters become brackish or salt.

[7] The ecoregion is home to fifty mammal species, including marsupials, bats, and murid rodents.

There is one endemic mammal species, the Fly River water rat (Leptomys signatus).