Rather than the treeless, limestone expanse we know today, the Plain was flush with gum and eucalyptus trees, banksias and other flowering plants now confined to Australia's east coast.
Scientists at the University of Melbourne used new techniques to date fossilised pollen and reveal the Plain's 'big wet' - a dramatic transformation in climate that occurred around five million years ago.
The finding sheds new light on the environmental history of the Nullarbor, a former seabed that was lifted above the sea 14 million years ago.