The map below comes courtesy of NASA's Earth Observatory and reveals the physical divisions very clearly........
Australia is the world’s smallest, flattest, and (after Antarctica) driest continent, but at 7.7 million square kilometers (3.0 million square miles), it is also the sixth largest country. Its low average elevation (300 meters, or less than 1000 feet) is caused by its position near the center of a tectonic plate, where there are no volcanic or other geologic forces of the type that raise the topography of other continents. In fact, Australia is the only continent without any current volcanic activity at all—the last eruption took place 1,400 years ago at Mt. Gambier (The maar eruption referred to in this posting) .
Prominent features of Australia include the Lake Eyre basin, the darker green region visible in the center-right. At 16 meters (52 feet) below sea level, this depression is one of the largest inland drainage systems in the world, covering more than 1.3 million square kilometers (500,000 square miles). The mountain range near the east coast is called the Great Dividing Range, forming a watershed between east- and west-flowing rivers. The crescent-shaped, uniformly green region in the south, just left of center, is the Nullarbor Plain with which we are now intimately acquainted!
The Australian continent is one of the oldest land masses, with some of its erosion-exposed bedrock dated at more than 3 billion years old. More than one-fifth of the land area is desert, with more than two-thirds being classified as arid or semi-arid and unsuitable for settlement. The coldest regions are in the highlands of Tasmania (large island at bottom right) and the Australian Alps at the southeastern corner of the continent, which is the location of Australia’s highest point, Mt. Kosciusko (2,228 meters, or 7,310 feet.)