The continental shelf extends north to Papua New Guinea and south around Tasmania , varying in width from about 30km to more than 240km. Just off the eastern coast the Great Barrier Reef extends north for 2000km from Southern Queensland to the Gulf of Papua. Enclosing about 207,000 square kilometres, the reef is an important marine ecosystem, a complex of islands and coral reefs containing many rare life forms. After Antarctica , it has the lowest precipitation of any continent. Mean annual rainfall is 465mm.
Vast areas are arid or semi-desert, unsuitable for settlement.
The average elevation is less than 300m, compared with the world's mean of about 700m. The main structural feature, the Great Western Plateau, covers most of Western Australia much of the Northern Territory and South Australia , and part of western Queensland. Uluru changes colour as the sun changes position in the sky. Nearly a third of the continent lies in the tropics, and the rest is in the temperate zone. The coldest regions are in the south eastern corner of the mainland, where the only regular snowfalls occur.