If Medicine is your desired course, you should try as many times as possible and to all the medical schools in Australia.
E.g. You should ideally sit the UMAT in each of the years of your Medical Science degree and sit the GAMSAT twice during that period.
You should apply to all medical schools in AUS to maximise your chance of getting accepted. Whilst, gaining entry into USYD is difficult and very hard, gaining entry into other medical schools in Australia will be slightly easier if not significantly.
I'd say that a good proportion of med-hopefuls in science based courses eventually get into Medicine through persistence and a good work ethic. But yeah, generally those that don't get in can either find a career using their degree, e.g. research in labs OR further study e.g. Hons or phD to become an academic.
OR they can use their degree to propel them into a postgraduate course e.g. a masters of Pharmacy, Physiotherapy, JD in Law etc. Because the degree is only 3 years it puts you in a good place to do a postgrad degree.