Although most of Australia is semi-arid or desert, it includes a diverse range of habitats from alpine heaths to tropical rainforests, and is recognised as a megadiverse country. Because of the continent’s great age (and consequent low levels of fertility), its extremely variable weather patterns, and its long-term geographic isolation, much of Australia’s biota is unique and diverse. About 85% of flowering plants, 84% of mammals, more than 45% of birds, and 89% of in-shore, temperate-zone fish are endemic. Australia has the greatest number of reptiles of any country, with 755 species. Many of Australia’s ecoregions, and the species within those regions, are threatened by human activities and introduced plant and animal species.

The federal Environment Protection and Biodiversity Conservation Act 1999 is a legal framework for the protection of threatened species. Numerous protected areas have been created under the national Biodiversity Action Plan to protect and preserve unique ecosystems; 64 wetlands are registered under the Ramsar Convention, and 16 World Heritage Sites have been established. Australia was ranked 13th in the world on the 2005 Environmental Sustainability Index.

Australian forests often contain a wide variety of eucalyptus trees and are mostly located in higher rainfall regions.

Most Australian woody plant species are evergreen and many are adapted to fire and drought, including many eucalypts and acacias. Australia has a rich variety of endemic legume species that thrive in nutrient-poor soils because of their symbiosis with rhizobia bacteria and mycorrhizal fungi. Among well-known Australian fauna are the monotremes (the platypus and the echidna); a host of marsupials, including the kangaroo, the koala, and the wombat; the saltwater and freshwater crocodiles; and birds such as the emu and the kookaburra. Australia is home to the largest number of venomous snakes in the world. The dingo was introduced by Austronesian people who traded with Indigenous Australians around 3000 BCE. Many plant and animal species became extinct soon after first human settlement, including the Australian megafauna; others have become extinct since European settlement, among them the thylacine.