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.
I am always amazed at news reports from Japan about how the sky is falling because their population is shrinking. For some reason no one ever says "on the plus side, Japan is one of the most densely populated places on the planet, a slow decrease in population will help improve quality of life measures and sustainability".
It's not fake money. It's likely not even "beyond means". The debts can likely be paid back. But it seems quite unlikely the level of spending can continue to grow.