My understanding of applying for postgrad med is that you apply through ACER (unless you have a Guaranteed Place, in which case you apply directly to the Uni itself). You put down a list of preferences with ACER for lots of Unis that offer postgrad (there are heaps of them), and it keeps working down the list until it finds someone who is willing to give you an interview (hopefully you get at least one). For more information, there is helpful information in the latter parts of this:
http://gamsat.acer.edu.au/images/documents/graduate_admission_guide_2011.pdfDon't forget you can do your GAMSAT in 2nd year and again in 3rd year and only report the better result, if you feel so inclined.
As for the number with "guaranteed places", in the 99.90+ stream, I have met about a dozen in first year Biomed. The average required is a 75% GPA (ie. H2A since GPA is calculated on the basis of H1, H2A, H2B, etc.)
There are certainly a lot more with 99.00+ in Biomed, with only some of them willing to take up a guaranteed full-fee place...
For quotas:
http://www.medicine.unimelb.edu.au/future/md/targets.html