Doesnt matter. As long as you have done first year chem, bio (maybe, it didnt help that much), and yr12 physics you're ok. The most important part is the 3 months before the exam, and what you do in those three months.
The physics in the exam was yr11 level last year. There is a lot of organic chem. Bio is pretty useless. You just need to know how to read data and interpret it quickly. Think of it as a hard science competition (similar to the UNSW ones you did in HS - but much harder)