Lots of people seem to want to know stuff about cut offs. Unfortunately, due to the process of combining GPA and GAMSAT and then offering interviews, there is no actual cut off. It is similar to what is happening with the Monash undergrad MBBS ATAR/UMAT stream at the moment.
Nonetheless, here are the average GAMSAT marks and GPA for the 2012 MD interview offers to give people an idea. I believe that this was a significant increase from the 2011 intake.
University of Melbourne
Melbourne ranked applicants for interview based on the weighted GPA and the GAMSAT score (where each of the three sections is weighted equally) to provide a rank list of applicants for selection.
As a result of using this method there were no discrete cut-off marks for either GPA or GAMSAT. Applicants who were successful in gaining an interview at The University of Melbourne for 2012 entry averaged an overall GAMSAT score of 65 and an average GPA of 6.6.
http://www.gemsas.edu.au/offers-2012/As you can see, GPA of 6.6 is pretty competitive. You would need mostly H1s and H2As.
According to my GAMSAT curve, 65 overall is around the ~91st percentile mark, but again it is difficult to make inferences because Melbourne does not use the overall score, but rather a score calulated by weighting each section of the test equally.
Just remember though, these are NOT cut offs. If you do better in one area, then you can do worse in another area. i.e. better GPA can make up for lower GAMSAT and vice versa.
Edit: Don't forget that the interview will have a fair bit to say about whether or not you will actually get in as well. It is worth one-third when they consider your final application.