Usually selection is based on a combination of your GPA (think of it as a kind of score that summarises your raw marks), your GAMSAT score and your performance in an interview.
ATAR will only count if you're looking for a Full-feed placed spot. So if you achieve a 99+ ATAR and chose to do study Biomedicine at Melbourne then say if you don't get a CSP placement in Medicine, you can get a guaranteed FFP placement, given that your ATAR was above 99 and you satisfied the other necessary pre-requisites.
If you get a 99.90+ ATAR then you are eligible for the Chancellor's Scholars program which guarantees you a place in the MD given that you pass an MMI which you can sit before beginning first year. The only thing is that you must maintain a WAM (just an average of your marks) above 70.
If you're set on doing Medicine, and I see you're subjects fulfil the pre-requisites, why not just apply for the Monash MBBS in your Year 12 VTAC application? That could save you a few years of time and definitely save you a lot of unnecessary stress.