The ancestors of the first Australians were among the first modern humans to leave Africa as part of the "Great Coastal Migration." When exactly the Great Coastal Migration left is debatable, but generally speaking this is between 50-60,000 years ago. As the name implies, this migration traveled along the coast of the Indian Ocean, from Africa into Arabia, and onward into the India and Southeast Asia. At the time, sea levels were much lower. The large islands of western Indonesia weren't islands; they were part of large peninsula called Sunda. Likewise, Australia, Tasmania, and Papua were one contiguous landmass called Sahul (in both cases "Sunda" and "Sahul" are modern names for these ancient landmasses, rather than ancient names that have lingered). But sea levels never got low enough to directly link Sunda and Sahul; the smaller islands of Indonesia remained islands. Here's a map for reference.

When the Great Coastal Migration reached the eastern end of Sunda, they had to island hop their way through these small islands to reach Australia. Boats or rafts would have been necessary for this part of the migration, and may have been necessary earlier if the Great Coast Migrants left Africa via the Horn rather than the Suez. But so far we haven't found direct evidence of what sort of watercraft might have been built at the time. In both Papua and mainland Australia the earliest reliable evidence for humans happens between 45-50,000 years ago. Historically speaking there's a wide diversity of small watercraft used by indigenous Australians (here's another map for reference); but 45-50,000 years ago is far to remote a time for this historical data to really be useful in telling us what sort of boats or rafts the first Australians used to make the final leg of their journey into Australia.