Consistent with the universally accepted theory that all humans have African origin, it seems humans somehow crossed the Wallace Line 40,000 years or more ago. They must have been at that time more technologically advanced than other humans. There is no evidence that other humans built efficient water craft that could cross the 50 km wide straight (at minimum sea level) for at least 25,000 years later.
I'm open minded on the subject and have no ax to grind. Without including Mobio and Population Genetics (along with Linguistics, dirt Archaeology and actual History) any comprehensive discussion would lose its way fairly quickly.
All the islands around Australia were settled by Melanesians, not Polynesians. Polynesian island settlement started north-east of Australia and gradually came south till the final settlement was NZ.
***@specul8: That the ancient North Coast is now largely submerged might work in favor of Underwater Archaeology turning up some info. It's a long shot, but what ain't that far back.