Australia has remarkably little land above 1,000 metres. Highlands (above 1000 m) occur chiefly at the southern end of the Dividing Range on the east coast (Fig 1). That is where orographic rainfall is most likely. Orographic rain is most common along the western slopes of the Snowy Mountains, and along the eastern side of the Dividing range from Sydney northward. Therefore the lowland west of the Dividing Range is quite dry.
The bathymetric contours offshore show how deep is the ocean, more than 4,000 metres, quite close to the shore, except to the north. The Torres Strait between Australia and Papua New Guinea is less than 40 m deep off Cape York, so that there would be dry land when the sea level dropped during the Ice Ages. Even Bass Strait between the mainland and Tasmania in the south would be dry, again allowing animal migration.
Fig 1. Topography of Australia and Papua New Guinea. Heights and depths are in m.