If you are not completing an Honours component as part of your degree, which you will not be since the Bachelor of Medical Science is not an Honours degree, and if you are not studying a Master of Research (which you are not), then you are not eligible for the University Medal.
You are, however, eligible for the Dean's Medal. This award requires you to have a GPA of 6 as the bare minimum. Where there are 50 or more graduands, you must place in the top 2% of your school's cohort (in this case, the top 2% of the School of Science), and where there are 49 or less graduands, you must have the highest GPA of your cohort, which again starts from the minimum requirement of 6.
Essentially, a GPA of 6 means that you are on a distinction average (i.e. you typically score marks between 75 and 84). The good thing is that having a GPA of 6 or higher places you on the Dean's Merit list for the academic year, which is also a good award. You can be on the Dean's Merit list as many times as the full-time duration of your degree. In the case of the Bachelor of Medical Science, you can be on the Dean's Merit list up to 3 times.
To answer your question, the Dean's Medal will obviously require a GPA significantly greater than 6 (closer to 7) since you will be competing with the smartest students in your cohort. What I am not entirely sure of is whether you will only be competing with students studying the Bachelor of Medical Science or with both the Bachelor of Medical Science and Bachelor of Medical Science (Advanced), which I believe is now called the Bachelor of Advanced Medical Science. If the latter is true, then it is likely that the Dean's Medal will be more difficult (but of course not impossible) to achieve given that the Bachelor of Advanced Medical Science is intended for high-achieving students and has an ATAR requirement of 95, considerably higher than that of the standard Bachelor of Medical Science.
It is therefore reasonable to expect that you will indeed need high distinctions in as many subjects as possible in order to have the highest possible GPA, allowing you to maximise your chances of receiving the Dean's Medal.
I hope this helps!