There was a time when Australia was uninhabited.
At some point a group of people settled the continent. They were not a seafaring people, so they came here by land, and settled also in Tasmania. Since Bass Strait is shallower than the seas around, they may have been able to move between islands via a land bridge.
Since no earlier people have so far been discovered, these people could have been called Aborigines. The Latin word 'ab' means 'from'. The Latin word 'origine' means 'the beginning'.
Some time later, another group arrived. This new group displaced the first group, perhaps through warfare or simply competing with them for food. The original inhabitants ceased to exist on the mainland, but it seems that the land bridge to Tasmania had disappeared, so the earlier group survived in Tasmania. And the people we call Aborigines would not seem to be Aboriginal at all.
Torres Strait Islanders
Much later a third group emerged from Daru in Papua. They used canoes to move to islands in Torres Strait, and made their way to Cape York. The Papuans were much more organised than Aborigines. They had gardens and had domesticated animals. They worked their way down Cape York, engaging with Aboriginal groups as they came. By the time white settlers arrived, they were well on the way to replacing Aborigines.
Captain Cook came here on the Endeavour in 1770, and thoroughly mapped the east coast. He determined that the west side and the east side were joined as a single continent. Limited contact with Aborigines led him to believe that the land was largely uninhabited, and he claimed the east of the continent for the British Crown. The British sent a fleet to settle Australia in 1788, preventing an attempt by France to colonise the continent. The colonisation by Torres Strait Islanders was limited to Cape York, and the Aborigines were able to survive on the mainland.