With an early enough POD, I'd say that Australia could support a maximum population somewhere between ten times and sixty times higher than its current population IOTL. After all, Australia as a whole, on average, is at least as hospitable and fertile as Rajasthan, and theoretically capable of supporting an equally high population density, provided that they have the right crop package. And using present-day Rajasthan's population density, of 200/km2, as a general guide for the maximum theoretical population density possible for Australia as a whole, we end up with a population figure of more than 1.5 BILLION people for the continent of Australia- comparable to that of India and Pakistan combined today, with just over half the population density. Yes, it does seem OTT from OTL's perspective on Australia, but it'd still theoretically be plausible. Even with a later POD, after the OTL European discovery of the continent, a far higher population would easily be plausible. The easiest way would be to simply have the British East India Company get in on the act with the initial colonisation period of Australia; instead of just having Australian settlement established through penal colonies and transporting British convicts over there, to provide indentured labor under penal servitude, why not have the British East Indian Company utilize the Indian indenture system to populate the continent at well, transporting Indian indentured laborers and convicts over there along with the penal transportation of people from the British Isles?
That way, you could easily increase the initial settler population of Australia more than tenfold compared to OTL. And with their own preferred crop package, bringing along their own far more suitable and productive crops for the Australian climate, such as grams, lentils, pulses, oilseeds and spices, they'd find it far easier to establish agriculture where the British convict settlers failed, as well as identifying indigenous Australian varieties of potentially lucrative cash crops such as cotton, agarwood and sandalwood which would further increase early settlement prior to the Gold Rushes. Then, simply allow population growth trends to continue from there, resulting in an Australia with a culture, history and demographics which parallel those of Mauritius IOTL, but on a far larger scale. IMHO, in such a TL, even with a POD as late as the 1800s, it'd be more than plausible for Australia to have a population of 100M or greater by the present day, without its standards of living, HDI level and GDP per capita being adversely affected. What do you think?