The ancestors of Aboriginal Australians had to make a sea voyage to get to Australia, as at no point was there a complete land bridge to get to the continent (Laidlaw, source details below). Dating wise the date of arrival keeps being pushed back, with remains found at Lake Mungo in relatively central Australia being dated to 40 000+ years ago (some controversy there in the dating - people argue to range from 40 000 years ago to 80 000 years ago), other sites at least 55 000 years old have also been found.

Either way Aboriginal people got to Australia via the sea, and some sort of island hopiing route, in numbers large enough to begin a cultural group (albeit with a large number of subgroups, with an estimated 700 languages and 750 000 people at the time of European colonisation), at least 40 000 years ago, and probably much longer, the Australian Museum says "at least 50 000 years ago". The Lake Mungo remains are inland and further remains found at the site show it used to be an inland lake that was used by large groups Aboriginal people in the area, over a long time, meaning the time it took Aboriginal people to spead out through Australia needs to be accounted for as well.

This/these voyages are not well recognised broadly as being as amazing as they actually are historically speaking, even within the Australian community, despite it being the earliest evidence of sea voyage and engineering/technology by a very long way. We don't have any idea how they did it at all technologically.

Sources: Australian Museum (linked), Australian Geographic (linked), I also linked you to Wikipedia on Lake Mungo and the pre history of Australia just to give you more info/somewhere to start reading if you're interested.

Also: Ron Laidlaw "Aboriginal Society before European settlement" in Tim Gurry (ed) (1984) The European Occupation. Heinemann Educational Australia, Richmond. p. 40.

We don't have info about the first sea voyage. The Polynesians were making sea voyages out of Taiwan 5,000 years ago, for instance.