Continents of the world

Australia

Australia 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 wider region known as Australasia.

Geography and nomenclature
The Australian continent is the smallest and lowest-lying of the Earth's continents, having a total land area of some 8,560,000 square kilometres. Though the Commonwealth of Australia occupies much of the continent and is often mistaken for being the entire continent, Australia and adjacent islands are connected by a shallow continental shelf covering some 2,500,000 square kilometres including the Sahul Shelf and Bass Strait and half of which is less than 50 metres deep.
As Australia the country is largely comprised of a single island, and comprises most of Australia the continent, it is sometimes informally referred to as "the island continent", especially for marketing purposes. Pedagogic simplification also results in many school children being taught that the continent and the country are synonymous, resulting in the misconception being perpetuated.
Prior to the 1970s, archaeologists called the single Pleistocene landmass by the name Australasia, although this word is most often used for a wider region that includes lands like New Zealand that are not on the same continental shelf. In the early 1970s they introduced the term Greater Australia for the Pleistocene continent. Then at a 1975 conference and consequent publication, they extended the name Sahul from its previous use for just the Sahul Shelf to cover the continent. A biologist, unaware of the terms used by archaeologists, suggested in 1984 the name Meganesia, meaning "great island" or "great island-group", applying it to both the Pleistocene continent and the present-day lands, and this name has been taken up by biologists. However, others have used Meganesia with different meanings: travel writer Paul Theroux included New Zealand in his definition and others have used it for Australia, New Zealand and Hawaii. Another biologist, Richard Dawkins, unimpressed with Sahul and Meganesia, coined the name Australinea in 2004

Geology
The continent primarily sits on the Indo-Australian Plate. The lands were joined with Antarctica as part of the southern supercontinent Gondwana until the plate began to drift north about 96 million years ago (mya). For most of the time since then, Australia-New Guinea has remained a single, continuous landmass.
When the last ice age ended about 10,000 years ago, rising sea levels formed Bass Strait, separating Tasmania from the mainland. Then about 8,000 to 6,500 years ago, the lowlands in the north were flooded by the sea, separating New Guinea and Australia.

Map :: Australia (Political Map) 1999, Courtesy of the University of Texas Libraries, The University of Texas at Austin