The real question is: Why didn't Melanesian taro cultivators cross the Torres Strait and filter down through Cape York to at least Cairns and possibly Mackay or Bundaberg?
Don't know, but the Torres Islanders had a generally superior civilization, especially in maritime skills. Some of the bigger islands off the coast not too far south were inhabited, like
http://en.wikipedia.org/wiki/Great_Palm_Island
Inhabited by about 200 people in Cooks first arrival.
Part of the issue is that the Great Barrier Reef offers few openings. Sailing expeditions by Polynesians or Melanesians would be blocked and the tendency is not to sail along it as long as Cook did. If they did, the tendency might have been to go to the mainland, where they would be attacked or absorbed by the Aborigine population.
Many of the uninhabited islands get covered in a hurricane (called Willy Willy in Australia, I hear) as being too low. Add to that a foam area of ten feet higher where no one can breathe. The storm surge alone can cover up to 65 feet higher than sea level, in recorded events, right over the hurricane eye. With enough overpopulation, one simply has the death toll covered up by higher or untouched islands, as happened in Polynesia. But it would be a death blow to a small population trying to make a go of it. Also, trade would be very difficult in a cloistered area as behind a nearly total barrier of reef, at least until the routes were discovered, which takes time.
So it may have been colonized, but died out and yet undiscovered.
http://en.wikipedia.org/wiki/Whitsunday_Islands
Also Aborigine settled. Not heard of any Aborigine oral history of guest invasions except south like Byron Bay.