Australia is the only country in the world, which occupies the whole continent. It is a country comprising the mainland of the Australian continent, the island of Tasmania, and numerous smaller islands. It is the world's sixth-largest country by total area.The total area is 7,617,930 sq km. Australia stretches from the north to the south to 3700 km; from the west to the east to 4000 km.
The continent of Australia is divided into four general topographic regions: (1) a low, sandy eastern coastal plain; (2) the eastern highlands, ranging from 300 to more than 2,100 m (1,000–7,000 ft) in altitude and extending from Cape York Peninsula in northern Queensland southward to Tasmania; (3) the central plains, consisting largely of a north-south series of drainage basins, including the Great Artesian Basin and (4) the western plateau.
Australian mountains have eroded over recent geological periods, and only about 6% of the continent is above 600 m; the average elevation is less than 300 m. The highest point is Mt. Kosciusko, 2,228 m in the Australian Alps of the southeastern corner of New South Wales; the lowest point is Lake Eyre in South Australia, 15 m below sea level. In 1983, grains of rock from Western Australia were dated at 4.1–4.2 billion years old, making them the oldest ever found on earth.
The most important river system, and the only one with a permanent, year-round flow, is formed by the Murray, Darling, and Murrumbidgee rivers in the southeast. The Murray River, Australia's largest, rises in the Australian Alps of New South Wales and flows some 2,600 km west and southwest. Several other rivers are important, but for the most part they carry great amounts of water in the wet season and are dry for the rest of the year. The largest lakes have no outlet and are usually dry. The Great Barrier Reef, the longest coral reef in the world, extends for about 2,000 km (1,243 mi) off the east coast of Queensland.