[Spocket]

Melbourne overtaking Sydney could easily happen.
Canada and Australia have many things in common no matter how counter-intuitive it may seem. One of the things they had in common for a long time was two large urban centers in the south-eastern regions. In Canada , Montreal was Canada's dominant city for almost all of the country's history. 100 years ago , had somebody suggested Toronto would outgrow Montreal , that person would have been laughed at. Now , not only is Toronto bigger than Montreal , it's almost doubly so. Depending on how you want to count it , it's more than twice the size of Montreal actually (although that's using a very large and otherwise impractical definition of Toronto's urban boundary) In 50 years Toronto shot up way past Montreal and is now on track to becoming a tier 1 city (although that's a very long way off if it ever happens at all)

In any case , the point is that both countries have similar populations , similar dispersal of population , and similar ...well everything but weather. If it could happen in Canada (whatever the reasons may have been) there's no good reason to assume it couldn't happen in Australia. Don't forget the principle of compounded interest : If the growth percentages stay the same , the actual number of people grows every year.