I think there is some confusion between Polynesians and Austronesians. Polynesians are one of the peoples who descend from the Austronesians.

It was another branch of Austronesian descendants of, who settled in Island Southeast Asia, and ultimately crossed the Indian Ocean and established themselves in Madagascar. They were cousins of the Polynesians if you want to think of it that way, though maybe second-cousins would be more accurate.

It is members of this Island SEA brand that did settle at least some of coastal Papua. In many cases they did mix, culturally, linguistically, genetically to varying degrees at different places with Papuan people already there. In the case of the Torres Islands, Austronesian arrivals did mix with Aboriginal peoples among the Torres Islands and the coasts of the Torres Straight. The people who live in this area are often referred to as being Aboriginal, but their culture contains a mix of elements from Norther Aboriginal groups, Papuan peoples, and Austronesians. This reflects the complex pattern of settlement which began in the area 40,000+ years ago-- back when the oceans were much lower and this entire area would have been above water. When the waters rose, some people adapted to their new maritime environment, especially coastal Papuan peoples and those who lived on what became the Torres Islands. The Austronesians don't appear till sometime between 3000-1500 BCE bringing with them new approaches to agriculture and navigation.

Otherwise, Polynesian peoples, such as those who settled on Aotearoa, would have found the crossing to Australia very difficult. The winds and currents are generally against you and even in the 19th century, ships found the crossing between the two to be potentially perilous to an unfortunate or inexperienced crew. It would be impossible to say that there was never an instance of a Maori crew finding themselves in Australia-- but considering there was no sustained contact or attempt to settle in Australia (no evidence of plant/aninmal introductions that might give us a clue, no oral traditions, no linguistic borrowings), its likely that if contact ever did happen in that direction, it was a one off affair. Either those who ended up in Australia were not able to make it home, or possibly they were not impressed with what they found and there was better land available in Aotearoa, or they could not make it back to Australia a second time. I would wager the first is the most likely if it ever did happen and there is no evidence that it ever did.