i don't actually do law but there are NO prerequisites if you want to do law at university. (this guy who's a law lecturer at macquarie uni came in to our school yesterday to talk to us about law). however, if you're doing a combined law degree i.e. law and engineering for example, there might be prerequisite subjects for the other major. Like for engineering you need i think 3 unit maths and physics (??) . this is important cos a lot of uni's offer combined law degrees vs. just bachelor of laws.
but as many of law degrees require pretty (scarily) high uai's (cos they're so popular), my advice is to take the subjects that you do well in or you enjoy doing in order to get the best marks you can to maximise your chances of getting in. don't play the scaling game where you take physics, 4 unit maths etc. etc. just because they supposedly 'scale' well and then end up hating them and also sucking at them... unless you actually like physics and 4unit maths or are good at them... ^^
anyway, you can always go to law school later i.e. doing graduate law school after you get a degree in something else. supposedly that's what the law guy who came to our school in did... he was an accountant for ages until he went back and did law school. law is a degree that doesn't need specialised knowledge first.