What precisely was that?

The time before people lived in Australia?

During the Ice Ages, just as there was the Bering land bridge between Alaska and Russia, there was a land bridge extending from Malaysia almost to Australia. There is ONE deep water rift separating Australia from Indonesia. Animals could not cross it. But people could.
That rift is where territory of the New Guinea birdwing butterflies and Australian Hercules moth stops. It is also where the Asian species moving south stopped. Species of all kinds were stopped there. Humans with canoes and outriggers could have crossed it.

In Australia at that time, they would have co-existed with a lizard much larger and deadlier than Komodo dragons. These huge lizards disappeared from Australia when people arrived.