You should join the Students' Association, screw the Union.
The Students' Association means you can get involved in issues affecting your degree, your campus and the Australian university experience. It also means access to the Second Hand Bookshop (cheaper textbooks than the Co-op!) and case-workers if you find that you've been unfairly marked/treated or failed a subject.
It's also cheaper to join ($15) and you get a free t-shirt.