While I'm not Australian, I've done my fair share of aptitude tests. The first when I was attempting to join the Canadian army reserve at 16. Being the lazy kid in school that I was, I did not score very well on it so alot of options weren't available to me at that time. It is important that you are well prepared in advance, but should be fairly simple for post-secondary students.
After graduating university, I also wrote several such tests while job hunting. One for the job I currently have as court sheriff, one for customs agency and two for police. They all share some similar features, but there are some difference between military and law enforcement aptitude exams. Know your high school math (algebra, fractions), english grammar and vocab, and be prepared to answer some common sense logic questions. In Canada we have something called the General Educational Development test, which is pretty much a high school equivalency exam. I found those tests as a good place to start preparation.
For law enforcement exams, memory and observation parts weight heavily towards your overall score, but I do not remember these playing a big role in my CF exam. I do remember however, that the military test had a good four pages devoted to spatial ability. You might want to consider that.
Last bit of advice: The exam questions are always alot harder than the preparation guides.