At the time of colonial settlement, the quokka was widespread and abundant, with its distribution encompassing an area of about of the South West of Western Australia, including the two offshore islands, Bald and Rottnest. By 1992, following extensive population declines in the 20th century, the quokka's distribution on the mainland had been reduced by more than 50% to an area of about . An estimated 4,000 quokkas live on the mainland, with nearly all mainland populations being groups of fewer than 50, although one declining group of over 700 occurs in the southern forest between
Nannup and
Denmark. In 2015, an extensive bushfire near
Northcliffe nearly eradicated one of the local mainland populations, with an estimated 90% of the 500 quokkas dying. In 2007, the quokka population on Rottnest Island was estimated at between 8,000 and 12,000.
Snakes are the quokka's only predator on the island. The population on smaller Bald Island, where the quokka has no predators, is 600–1,000. At the end of summer and into autumn, a seasonal decline of quokkas occurs on Rottnest Island, where loss of vegetation and reduction of available surface water can lead to starvation. This species saw the most significant decline from 1930 to the 1990s, when their distribution was reduced by over half. The quokka markedly declined in its abundance and distribution in the early 1930s, and this tendency has continued till today. Their presence on the mainland has declined to such an extent that they are only found in small groups in bushland surrounding Perth. In late 2024 a new quokka population was discovered in the Perth Hills. It is the first time that quokkas have been photographed by the general public in the Perth Hills and is an important finding for conservation of the species. Their exact location will remain confidential. The quokka is now listed as vulnerable in accordance with the IUCN criteria. == Conservation ==