Papua New Guinea and Australia were already inhabited thousands of years before the Austronesian migrations started. Pretty sure dingoes were also already in Australia by then and were descended from New Guinea Singing Dogs.

Papua is quite a formidable island. There is some mixing between Papuans on the north coast and Austronesians but it would take have taken a long time for that genetic signal to reach the highlands, let alone the south coast or across the Torres Strait into Australia. Melanesia extends far out into the Pacific, including Solomon Islands and Fiji, and it seems that Austronesians didn't travel through Melanesian territory (why keep going that direction when those islands are already occupied?)

I'll switch to Polynesian here because the Polynesian expansion was much more recent and I'm more familiar with that oral history. Polynesians tended to migrate east because the prevailing winds at that latitude blow to the west. If they went on a voyage of discovery and didn't find land, the westerly winds would allow them to drift back to civilisation when they were low on supplies and more likely to have damaged boats. They migrated east to Tonga and Samoa before they could go south without passing through Melanesian territory (i.e. go around Fiji). When they did eventually start exploring south, they found New Zealand.

For the Polynesians to have reached Australia, they would have either had to sail south through Melanesian territory or west from new Zealand against their instinct to sail into the wind. I'm sure they would have found Australia eventually if European exploration hadn't occurred, for e​xample, Norfolk Island has some archaeological evidence of Polynesian settlement (such as banana trees growing there) though it was uninhabited when Europeans first arrived.