How plausible would it be for the Australian Aboriginals to succeed in rising up against the European colonists and either reach acceptable demands for representation, or even defeat them and kick them out of the continent?
POD starting since the moment colonization in Australia started.
Rather unlikely. The Aboriginals were not a united people but many thousands of fairly insular tribes, with hundreds of languages, making coordination difficult even where they recognise their mutual peril. Moreover, there weren't many of them and never concentrated in such large numbers as to effectively decapitate whatever force the British might be operating with. Even the highest estimates only put the Aboriginal population at 1 or 2 million across the whole of the continent. Disease ravaged them(though not quite to the same degree as in the Americas given they had some preexisting contact -and so likely some exposure to the diseases- with the civilizations of the Indian Ocean), and gathering a large force of warriors together would only vacillate the spread of those diseases. Finally, they sorta did rise up, repeatedly, against the colonial authority IOTL. It just wasn't enough, and hundreds were slaughtered as a result. And the British had a lot of experience by that point (1788) combating much mored advanced natives, be they Indians or Native Americans, so the Aboriginals frankly didn't stand a chance.