During this time period Australia had After being recycled many times over much of the history of the Earth, with no new rock added by volcanism, the soils of arid Australia have been weathered and leached more than any others making them the most nutrient-deficient soils in the world.
And most of Australia spent many millions of years covered by seas.
As a result of this, poor soil, arid and unpredictable climate over most of the continent, the plants, and therefore the animals that depend on them, have evolved into a unique flora and fauna.
Another factor in the formation of fertile soils that was absent in Australia was glaciers.
During the ice ages of the last 2.6 million years, glaciers covered much of the Northern Hemisphere, grinding unweathered rock into fine particles that became soil enriched with new minerals when the glaciers retreated.
There are 3 major structural components, the stable Western Shield, the gently warped Central Basin, and the ancient orogony of the Eastern Uplands, which have been rejuvenated by differential uplift in Tertiary and later times.
The result is the vast plains and plateaux of the Australian landscape.
There are not many areas above about 1700 m, and even at Mt Kosciusko, the highest point on the continent, it is less then 3000 m above sea level.