I do Believe in a Big Australia

I don't necessarily believe in a big China, Europe, America etc. On balance, them shrinking would leave a nice vacuum for us to fill. I have absolutely no doubts about "sustainability" in an Australia specific sense. If there is an upper limit on our population, it would be near the population density of Europe or the US, which means I believe we can sustain a population of 200 Million as easily (or as difficultly) as Europe or The US sustains its current population. Within a couple of decades, there may well be several developed countries with a fairly rapid decline in population. They will not, on average be doing particularly well and citizens of those countries with ambition, will very likely try to move somewhere like Australia.