There has never been a land bridge to Australia. Getting there involved many sea crossings, no matter when you place the event, no matter how low the sea level fell as water was locked up in glaciers.

Look up "Wallace Line". Animals, likewise, could not walk from Australia to Asia, for the same reason. Thus, we do not have kangaroos and wallabies in Asia, though they are in New Guinea.

https://en.wikipedia.org/wiki/Wallace_Line