I think the only way for the British settlers to take Aborigines seriously as people is for them to be able to put up a fight (not necessarily too much of one - this could easily tip into the genocidal side of war, but more of one). If the abroriginal peoples had had more organisation, more moveable wealth, and a slightly higher level of technology that might well have worked (as it did for the Maori), leading, as Melvin points out, to recognition of their title to the land which they then can sell, rent, or cede to the British.
Maybe if we up the level of outside contact? THere were sporadic interactions with Malay fishermen and some southern Chinese along the northern coast. WI the Aborigines had brought something valuable to trade, starting say around 1500 (what is there in the area? I'm instinctively thinking gold or opals, but Australia is pretty big and I'm not sure the northern groups or Torres Straits Islanders even knew where to find them. Maybe a kind of plant the Chinese like to eat is a more realistic alternative)? Then the northern tribes would get valuable trade goods (metal implements, textiles, glass vessels, pottery, maybe guns) and could in their turn trade south, slowly permeating Australia with 'modern technology'. I don't think the Aborigines would develop into a technologically advanced society, but they would get exposed tomore diseases, thus develop immunities earlier, get more used to the sight of guns and metal, come to develop longer-ranged structures of exchange and dominance with a more hard-nosed 'I want that' approach to life, and might even get a few domesticated animals out of the deal (I dunno. goats? pigs? sheep? I doubt the Malays would be in a position to sell them horses, nor would their customers want them). Once the central tribes notice that their northern couasins will give them valuable glass, metal, and cloth for boring old bits of stone they'll also join the network, and once they find out what the Malay pay for gold and opals they may invent the concept of trade war all of their own
Once the British enter this territory, they will face more warlike, more technologically advanced, less scared-of-strangers populations who will give them a run for their money. Of course the Brits still win, but there'd very likely be some sort of treatys and the aborigines would generously get to keep all the bits of Australia nobody else wanted (until they find gold, copper, coal, cobalt, or what have you there). It would at leasttranslate into a legal position to take in court.
MInd you, this POD will most likely butterfly away the Brits, making either the Portuguese or the Dutch the first European invaders. You don't go buying fine opals and gold from Malay traders for years without wanting to find out where they come from. Maybe it is better to limit the trade to something only the Chinese would eat
