Boats! The earliest population in Australia is very early indeed, the positive evidence showing 40,000-60,000 years ago, depending on who you ask. During this time, glaciation meant that sea levels were considerably lower, and the Eurasian continental landmass extended down past the modern island of Java. But after that, along the so-called Wallace line, the only way to get further south was by boat. That humans demonstrably did arrive in Sahul at this time means that this is actually the earliest positively known example of voyaging, boat use and colonization by humanity.

While obviously this question can't be answered in the way the OP wants it to be answered (Ie, with respect to their personal beliefs about the history of earth which have no relation to history, anthropology, geology, or really any modern scholarship), I'm leaving this thread up so that people who care to can answer the question.

Since no one is responding... I have read a fair bit about the situation. No there is no written records pre european intervention, only oral histories. Amazingly some of those oral stories discuss events > 15K years ago. One of the groups living near me have stories about the post glacial sea level rises for example (Green Island, off Cairns).

In terms of their beliefs about where they came from, there are a mixture of "dreamtime" stories, although these stories differ between groups significantly. Generally the beliefs normally concern totems and lanscapes from which the people derived from. To be honest as an outsider of their culture it is hard to translate the sense of the stories meaningfully. The more I learn the less I know. Also I believe we often are impeded by cultural taboos on revealing all the information to outsiders.

I cant really reccomend any particular book. David Gulipil published a book many years ago that was a collection of traditional stories. I recently read a book called Aboriginal Men of High Degree; Initiation and Sorcery. That book from memory had details on a number of groups stories and analysed commonalities. I generally find a dearth of real information unfortunately. We have lost so much information, only a few areas seem to have good records.