No proof has ever been turned up for human habitation of Australia prior to the initial peopling of Sahul (the larger continent of which Australia is part).

So; what if? I haven't heard of anyone trying to prove that people were living on Sahul prior to the arrival of the peoples 40,000 years ago who would became Australian aboriginals. The likelihood of it seems small; landscapes and environments change dramatically when humans enter them- this is one of the ways in which human settlement on Sahul is attested- there are no records of dramatic anthropogenic change in the area prior to roughly 40,000 years ago.

So; its doubtful anyone can *disprove that there was anyone there before, but its more unlikely that anyone can *prove there was someone there.