Fair enough. I wouldn't have thought the growth rate would be so high considering the millennias that the Aboriginals have inhabited the land and were only a few hundred thousand in number when Europeans showed up.
It was still around 750K despite most of Australia being desert, a lack of large game animals, poor shipbuilding skills, and very poor connections and trade routes to the larger world. The thing is the Maori would be bringing resources Australia did not have--domesticated animals (dogs and pigs) and domesticated plants, which would let the Maori population rapidly expand. It helps that Maori agriculture is more productive in Australia than New Zealand because of the climate.
What do think the population might be if Polynesians settle around a thousand or so years earlier when their migrations took them closest to Australia in OTL? Larger surely but would it radically change things? I don't know enough about ocean currents and trade winds to know if sustained contact would be plausible.
In that case I think most of Eastern Australia would be Maori with the remnant Aboriginal groups in that area living in dense jungle in the north (which also has very poor soil) or certain dry interior highlands on the fringe of the desert, plus parts of western Tasmania. Just checking the map shows there's maybe 750K-900K km2 that would make for decent settlement in eastern Australia, and all but Tasmania is going to have conditions akin to the most densely populated part of precolonial New Zealand (area of modern Auckland and regions north). Plugging in New Zealand's precolonial population density gets me 355K people in that area. It might be a bit higher. But a lot depends on political complexity, that is, getting the manpower to reclaim land, build flood protections, and irrigate dry land.
A higher estimate could be up to 1 million people (about the population density of Indonesia in 1 AD). But IMO it's going to be somewhere in between unless they become part of the Malay cultural realm and get Malay, Javanese, etc. settlers, envoys, missionaries, etc. and start setting up true state societies which have iron tools, more advanced construction and engineering, livestock like horses and water buffalo, and the wide variety of crops used in those societies. In the more remote parts of Indonesia, this didn't really happen until the 13th century or so when the various chiefdoms built a sturdier society than what came before and created strong local states like Ternate, Gowa-Talloq, etc.
What would be interesting is these connections to the Malay world eventually expanding to Australia (maybe they trade
Tasmannia peppers?). Perhaps it's in the 16th century or so on the eve of European arrival to the region. You might have a turbocharged Musket Wars-esque scenario brought about by introduction of new crops and tools, trading spices for weapons, and above all, arrival of Christianity and Islam. I'd say that would totally fragment the local Polynesian ethnicity but if they arrived in 300 AD than by 1600 AD they'll be at least several different cultures with different lifestyles and speaking quite different languages.