It's by no means impossible to gain post graduate education at a top US university, the main problem is the cost. I would say if you are motivated to get in, and are undertaking a degree at a top Australian university then there is nothing stopping you. As the American undergraduate courses are 4 years, you will require more than an undergraduate degree from AUS on most occasions, maybe a masters or something like a diploma in mathematics.
As to cost, it's tough being an international. You will probably have to apply to what is called an "need blind" college, which means they accept people before looking at their requests for financial aid, therefore such requests will not disadvantage you. If you were to apply to a non need blind college and were to request financial aid, being an international you would want to have some OUTSTANDING past academic results to get in.