Australia at 25 million - how did we get here?

Happy World Population Day!  On this day when we focus on all things population, why not have a look at the history of Australia's population since 1788.  There's no doubt that Australia is recording strong population growth in the current era, and the ABS estimates that Australia reach a population of 25 million in the coming weeks.  But how did we get here, and when did we reach other population milestones in the past?