1. I think it is because Australia is not fertile enough, the sea near Australia is rich with fish and it has been a traditional fishing ground for Indonesian fisherman. They seems to be very satisfied with Indonesian archipelago and start building many kingdom there.
2. There are multiple migration to Indonesia and Malaysia. The first migration has common look like African people and Malaysian called them orang asli (original people) but in Indonesia we dont have them. The second migration comes from Yunnan China and the last one from Taiwan. This theory have been approved by recent DNA finding conducted by Japanese university of several ancient body remain where the oldest has similarity with Andaman island people (African feature), the younger body have DNA similarity with Yunnan people from China, and the recent one has similarity with indigenous Taiwanese.
3. Pasific islander most probably mixed of Taiwanese and bigger people like Papuan.
Just look on how close Chinese Han people with Indonesian/Malaysian people DNA
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Recent Indonesian DNA also has many variation from many part of the world like Middle East and South Asia,
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