A two part question, each largely unrelated to the other.
New Guinea
New Guinea was explored, settled and traded with (though not dicovered) by Polynesians and proto-Polynesians. The Lapitans were the direct ancestors of the Polynesians and settled large chunks of New Guinea. Do a Google search for “Lapita People” or “Lapita Pottery” for more details.
Moreover Polynesians from areas west of New Guinea were regularly trading with New Guinea into the 18th century at least.
In addition the Polynesian Maori of New Zealand are genetically a blend of true Polynesian and a recent influx of Melanesian ancestry, most probably from from New Guinea.
So to say that the Maori never found New Guinea is just plain wrong. They discovered it very, very early in the piece and have continued to have full knowledge of its existence right through to the present day.
Australia
Nobody knows whether Polynesians ever made contact with Australia. There is some fragmentary evidence, such as the isolated use of shell fishooks in central coastal NSW and the use of wooden broadswords in Northern Queensland which suggest Polynesian contact, but nothing conclusive.
It might seem unlikely that the Polynesians never made contact with Australia when they had such a long and intimate contact with New Guinea, but the Great Barrier Reef would have both confused their navigation techniques and made successful landafall very difficult so it’s certainly not impossible.
More likely the Polyneains made landfall in Austyralia but never managed to establish a colony. Jared Diamond devoted briefly considered why this should be in G,G&S and Flannery devoted alot more time to it in “The Future Eaters”. The simplified answer is that Polyensians were were Tropical agriculturalists and fishermen, and Australia simply isn’t suited to wet tropical agriculture and fishing. The climate is too variable, the soils to poor and the ocean to unprodutive to allow the Polynesian way of life to be viable. Any Polynesians who did make landfall wouldn’t have been able to establish a viable colony. We know that that numerous Indo-Malayan farming colonies in Northern Australia had to be abandoned and the English colonies in Southern Australia almost failed numerous times and did fail multiple times in Northern Australia, and these were people arriving in number and in continued contact with the homeland. For an isolated Polynesian settlement party the odds of actually developing a viable farming/fishing community in Australia would have been virtually nil.
Aboriginal hostility may have been a factor, but only in the North. In the South Aboriginal reations to settlers seems to have been mainly aloof curiosity rather than outright agression. In the North people were abit more agressive, but that was a learnmed reation due to prolonged contact with seafarers/raiders from New Guinea, The Torres Strait and Indo-Malaya.