However Australia has such a large amount of land, and such a small population that forecasted worldwide population drop won't affect Australia so much. It's such a desirable place to live that immigration at the current rates will be sustainable regardless of the rest of the world's population growth rate. Whether or not our cities can keep up with the growth and maintain their high levels of livability is another question.
I'd also question your statement about population drop on the next 15-20 years. All forecasts I have seen are that the world's population will continue to grow until the end of the century. If the world's population were set to start shrinking in such a short timeframe, I'd think it would be a huge talking point in any discussion about sustainability and global warming.