Australia is an old continent, home to the Earth's most ancient rocks. By 542 million years ago most of the present Australian landmass had been formed, though it was joined to the future Indian and Antarctic landmasses and lay far to the south of its present location. The Tasman Sea began to open between 80 and 60 million years ago, and the final separation from Antarctica commenced. So began the long period of isolation in which Australia's distinctive flora and fauna would evolve. At this time, much of Australia was covered with well-watered, species-rich rainforests. The wet conditions produced deep weathering and leaching of the land surface, contributing to the characteristically infertile soils found over much of the continent today. Nowhere but Tasmania and the Snowy Mountains in the south-east would soils be renewed by glacial activity, and only along parts of the eastern seaboard and a few other small areas would volcanic activity produce young, fertile soils comparable to those in other continents.

Australia drifted north and by 11 million years ago it was over 2,000 kilometres from Antarctica. As the circumpolar ocean current began to flow, Australian climates became cooler, drier and more seasonal. Rainforests gave way to woodlands, shrublands and grasslands. Fires increased. Eucalypts and acacias, both present since at least 45 million years ago, became prominent. Western Australian plants, previously separated from eastern species by an inland sea, became isolated by the arid Nullarbor Plain, producing highly distinctive flora.

Then, from approximately 2.4 million years ago, the Earth entered a period of rapid climatic change, alternating between cold and arid glacial phases, and warmer and wetter interglacial periods, with long intervals of cool and dry conditions. Sea levels fell and rose as water was frozen in or released from polar icecaps. Seasonal aridity in the inland was well established by 700,000 years ago with salt lakes and shifting dunes found in some parts, while others supported a flora dominated by sheoaks, saltbushes and grasses.