During the last ice age around 10,000 years ago, ocean levels were much lower and Tasmania was linked to Victoria via the Bassian Rise.

 

At this time, Port Phillip Bay was a dry lowland basin and the Yarra River meandered down its eastern flank before turning west and entering the sea at Port Phillip Heads.

 

About 8000 years ago as the ice age finished, glaciers and snow ice began to melt and ocean levels rose around the world. As this happened, the Port Phillip Bay basin flooded and the Yarra River mouth retreated to modern day Williamstown.

 

At the same time, the Bassian Rise land mass was submerged, and Tasmania became isolated from the mainland mass to its north.