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

My understanding is that while there were substantial "land bridges" at the time Australia was initially colonized, nevertheless sea crossings were still required. But it's a bit tough to follow honestly, do you have a clearer picture?

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