It seems inconceivable that the Polynesians managed to find Hawaii way out in the middle of nowhere, and sail right on past Austrakia to New Zealand, and somehow missed this enormous contingent. I mean, it's right there. Plus, looking at a map of the Torres Strait, it's clear that Queensland and Papua New Guinea are right next to each other. The whole eastern coast of Australia is very lush and habitable - so what gives?
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I had a really long answer written up then accidentally closed the page but here are the cliff notes:
That's about where I got up to. So you can see that there are good reasons why Australia wasn't settled by Polynesians as well as by Melanesians up until around 2000 years ago. Why they haven't settled since is more difficult to answer, since Austronesians from the southern coast of New Guinea (speakers of Central Papuan Tip languages, e.g. the Motu) traded with Torres Strait Islanders and possibly mainland Australians as well, so they clearly knew about it. Maybe they didn't feel the need to or tried and met resistance from native populations ... who knows.
Of course, if you go back far enough (60,000 years or so), Australians and Melanesians are the same people and during various ice ages in the subsequent years, New Guinea and Australia have been connected by a landmass into one continent (Sahul). So in more distant prehistoric times there was probably a lot more movement and interaction between Australians and Melanesians including people from New Guinea settling in Australia, but at least linguistically this would have happened too long ago for linguists to easily detect with current methods.