GCSE grades are not as important but you'll probably need at least B/C in English and Maths as a general requirement.
If you apply for commonwealth supported places you will need at least AAA/AAB for the Go8 universities. If you ain't looking at Go8, the grades will be lower but for all of them, you can check the university websites. On the websites, go to the domestic student section and look for your course. The grades should be listed in UAI but you can roughly convert it to A-Levels. 99+ is like AAA, 96+ is close to AAB, 93+ is like ABB and BBB is around high 80s and low 90s. Its a rough idea and i can't really find any resource that give you accurate info since the UAI changes every year.
Look at this page and scroll down to the bottom:
http://www.satac.edu.au/uniwebS2/sect02/year12sub.asp
It shows the UCAS points you need but its still only general.
Also, the prerequisites are science subjects but each university is different, check their websites. I think in most universities, medicine is a post-grad subject so you need to do a relevant bachelor degree first. Among the Go8 universities, U Syd, UQ as far as i know are really "sciency".
As to their education, its varies from state to state and its state-wide exams. It doesn't matter much to you i think. You should e-mail each university your specific questions and they'll give you more reliable answers.
You should also e-mail the universities