In geology and biogeography, Australia (also called Australia-New Guinea, Sahul, Meganesia, Greater Australia, or Australinea) is a continent comprising (in order of size) the Australian mainland, New Guinea, Tasmania, and intervening islands, all of which sit on the same continental shelf. These landmasses are separated by seas overlying the continental shelf — the Arafura Sea and Torres Strait between Australia and New Guinea, and Bass Strait between mainland Australia and Tasmania.
When sea levels were lower during the Pleistocene ice age, including the last glacial maximum about 18,000 years ago, the lands formed a single, continuous landmass. During the past ten thousand years, rising sea levels overflowed the lowlands and separated the continent into today's low-lying semi-arid mainland and the two mountainous islands of New Guinea and Tasmania.
Geologically, the continent extends to the edge of the continental shelf, so the now-separate lands can still be considered a continent. Due to the spread of flora and fauna across the single Pleistocene landmass, the separate lands have a related biota.
New Zealand is not on the same continental shelf and so is not part of the continent of Australia but is part of the submerged continent Zealandia and the wider region known as Oceania.
Australia is classified by the Australian Government Department of Foreign Affairs and Trade as the world's largest island and the world's smallest continent.
Mainland Australia, with an area of 7.69 million square kilometres, is alternately regarded as either the Earth’s largest island or smallest continent. It stretches about 3700 kilometres from north to south and about 4000 kilometres from east to west.
