There are two ways one could achieve this:
First: one could of course have a prolonged contact with the Makassans: IOTL, the Yolŋu learned how to make superior spears and canoes from the Makassans (they also got smallpox in the 1820s; too late to be of much help, immunity-wise);
the Yolŋu may even have elements of Islam into their traditional religion.
The Makassan trepangers perhaps were
too distant from the Yolŋu: IMHO, while it was of course decent and humane, even admirable, of them to respect Yolŋu (and one would assume any other Indigenous Australian Peoples') customs and land, needless to say frankly magnanimous in comparison to what the Westerners as a whole did to (from what I can tell) all (and certainly at least pretty much all) Indigenous Australians in the form of what amounted to nothing short of genocide, nonetheless the Yolŋu - and the precolonial Indigenous Australian population as a whole - could certainly have used ocean-going sailing ships, gunpowder, steelmaking, writing, intensive agriculture and state-level social complexity, all of which the Makassans had. Contact with the world beyond would certainly have made it quite possible for the Indigenous Australians to stand a chance against the Western Invaders.
Or: As Jared has already done, one could have a more or less fully indigenous development of culture. Now, since he has done such an excellent job with the Lands of Red and Gold, I will just mention a few things:
The Gunditjmara at Lake Condah could provide a useful starting point for indigenous state societies to form; the eel farming (more like ranching actually, but you get the point) in the area reminds me of Mosely's theory on "The Maritime Foundations of Andean Civilization", so it could work.
That said, the other good place (perhaps better) is North Queensland because: New Guinean crops can grow there. Now, I know New Guinea isn't part of the Indigenous Australian cultural area, but
the two lands were connected during the Last Ice Age... Anyway, the relatively wet tropical climate would be far more suitable for New Guinean crops - taro, winged beans, certain beefwood trees, bananas, a species of toddy palm, nipa palms, pili nuts, black-fiber palms,
Pandanus palms, sugarcane and were all first cultivated in New Guinea. With the addition of native North Australian crops such as macadamias, Australian "limes", zig-zag vines and wattles, and livestock such as eel-tailed catfish, redclaws, stingless bees, honeypot ants, jade perch, Australian Giant Freshwater Prawns, and of course emus, dogs and pigs (the latter two from New Guinea). Hunting and fishing would also be important, especially of macropod populations.