Different research area in undergraduate and graduate
1) Bachelor's degrees in the US are generally less specialized than those in other countries (though this is less true in engineering).  It is quite common for people to enter graduate school without well-defined research interests at all and to make decisions only after a year or two of coursework and trying out different labs.

2) Any large department at a large major research university in the US (such as the University of Michigan) will have experience with graduate students from Australia and may very well have faculty who were undergraduates in Australia.  They will evaluate the University of Wollongong as comparable to similar caliber universities in the US.  It is true that, because some (but not all) funding streams and accounting mechanisms give priority to domestic students, that US citizens or permanent residents may have an advantage in admissions, but that has nothing to do with where you were an undergraduate.  (In particular, if you are a US citizen, having gone to the University of Wollongong wouldn't hurt you.)