Moved to General Discussion boards. The University Results sub-forum is strictly for discussion re: end-of-semester university marks and grading, so please be mindful of where you post.
As for your questions, there are many things to keep in mind. Above all else, you need to be aware of the fact that the undergrad you do has no bearing on your ability to get into med at Melbourne, apart from satisfying the pre-requisite subjects in anat/biochem/physiology. There is absolutely no selection bias towards biomedicine students, so a candidate from BSc is just as likely to get an offer as a biomed student with the same scores. On top of that, it's not really possible to give you the statistics or numbers that you're after, because they don't exist. Not everyone in science wants to medicine, and of those that do, not all of them apply with competitive scores. The same is true for biomedicine - there are many that have other interests (research; physio, etc.) or that don't have strong enough GAMSAT/GPA combos to apply with. Because of this, it becomes complicated to say statements such as 'x number of people out of 500 get in'.
Additionally, nobody can really tell you with any objective measure how difficult it is to have a high enough combo score to qualify for an interview, because this difficulty is subjective to each person. I would remark, though, that it's not some impossible pipe dream reserved for the gunners with a 7.0 and 100 GAMSAT. The reason that there are no scores that anyone can give you that are definitely indicative of 'cut-offs' is because a lower GPA can be remedied by a stronger GAMSAT and vice versa. Similarly, it's important to note that these scores are subject to change from year-to-year, and that as of late, Melbourne uni are placing increased emphasis on the MMI process (now worth 50% of your ranking).
Two more notes about grades: (1) As vox mentioned, it is possible to complete further study to boost your GPA, but you shouldn't rely on this. Most universities (including unimelb) only consider undergraduate study (Bachelor degree, including honours) into your wGPA calculation. Boosting your grades isn't as simple as getting a higher GAMSAT if you've already finished your degree, because it means doing honours (which might not take effect at mid-year) or another undergrad degree. (2) The weighting for each respective year is now 1:2:2, so second year is worth double that of first year, and equal to third year.