I think the gap between Asia and PNG (then connected to AU) has got down to 90km. Doable with bamboo rafts? Viewing a mountainous island from an elevated position would give them reason to attempt it. Timor to PNG is suggested as the water crossing needed.
While there is considerable genetic diversity between Aboriginals as a whole, the diversity of individual communities is often quite low, and in response many communities evolved elaborate kinship systems to avoid too much intermarriage.
