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.
I've looked into it, you should be fine getting in with just an undegraduate degree. However, you'll definitely need to have done honours. Moreover, you'll need a very solid honours result - most likely first class honours (i.e. an average of 80+) or upper second class honours (this is 70+ at ANU, but depends on the uni...).
Financial aid is the hard part. You pretty much need to win a university medal (i.e. get first class honours and 95 average or so) in order to get some sort of scholarship.
That would be my goal. You have to get AMAZING scores. I know a guy who's doing that who was a physics lab demonstrator for me this semester. He maintained a solid 90 average throughout his course and was on a whole bunch of research projects and work experience etc. and got admitted to a Masters of Engineering at MIT. In conclusion, very hard but DOABLE.
I don't get it... is 90 really that hard to achieve in university? Is it like 90% or 90/90? 
An HD (=80) is the equivalent of an A+. Unlike high school, uni exams generally aren't made for it to be likely for anyone to get 100%, though of course with guys like Ahmad it can still happen. To consistently get marks above 90 means you need to be both very smart and put in a lot of effort.
90 is extremely hard to achieve in uni. Even in arts! 
Harder in Arts, definitely. Though my average for the two Arts subjects Iv'e done is higher than my average in maths

Yay intro to Latin!
Arts subjects are hard to score highly in because there is no such thing as a right answer. Unlike most (if not all) sciences, it is pretty much impossible to get 100% - the "perfect" essay does not exist!
Yeah pete shearman, a BOSer, said his dad, a prof of political science at UMelb, had given 92 as his highest mark in 20 years of teaching. Also Pete, who seemed pretty decent himself [and got a premier's award for English] got [i think] 85 and 88 in UMEP History, wheras Ahmad and Neobeo got 97 for UMEP Maths
Pete Shearman lol. He was a friend of mine at school. He didn't top UMEP History, btw...
But yeah, in areas like history, english, politics etc (basically anything predominantly essay-based that can be quite subjective), it is extremely unusual for even the best of students to get near a mark of 90.