For Australia to host a native population higher than OTL's modern Australian population probably requires quite an ancient agricultural PoD - as per the seminal LoRaG - though perhaps some more 'recent' changes i.e. last couple of thousand years could see Australia absorbed in the South East Asian cultural sphere, with resultant connections to the wider Afro-Eurasian networks. I cannot see how in the last thousand years or so Aboriginal Australia could transform itself quickly enough to reach such an increase (some 25-fold), though would be very happy to be shown how.
In terms of 'modern Australia', the last 230 years, it's relatively simple to achieve a larger population. Avoid the world wars and the population is probably over 30 million. Modest increases in population during the gold rush could translate to several more million people today, and the increases don't have to remain modest. An early WA gold rush is a very easy PoD, and anything that lessens the impact of the 1890's depression will have a pretty positive impact on population growth. Early federation might help over time. Avoiding the very strict version of the WAP would help - if Italians and Slavs were let in from 1900 Australia could have easily gained half a million more migrants by WW1. There's lots of possibilities. 'Carrying capacity' is not a strict term, but Australia could probably sustain 50 odd millions without substantial alterations to the current set up. Beyond that and practices might need to shift in some areas.