Well, the Australian population has passed 25 million. Twenty five million! When I opened a primary school geography book in the 1970s, it was less than 14 million. Older readers -- most of my readers -- might remember when it was less than 10 million, before the end of the 1950s. You’d have to be 80 or over to remember when it was in the 7 millions on the census (which excluded Indigenous Australians, remember) -- a genuinely small Anglo-Celtic outpost at the bottom of Asia, settled in part so as to make it easier to colonise Asia, and thus scaring itself to death from the 1870s onwards that Asia might reverse the process.
Twenty five million is an absurdly low number of course. Prior to any consideration of whether the continent could take that many more, reflect on how singular and ridiculous this place is. Forty percent of the population are in two cities. Even if you look at where people really are, we are enormously empty. Draw a line from, say, Portland, Victoria, to just above Brisbane, and the resulting "green triangle" will contain about 70% of the population, and still be largely empty. That triangle is about the size of France and Germany combined, or mainland South East Asia, each hosting nine-figure populations.