For most unis you don't need any subject prerequisites (eg. You could do an arts degree an still do post grad med at most unis). You will require some first year chem and biology knowledge, whether obtained through taking those subjects or learning those in your own time, for the GAMSAT though. For Melbourne however you need second year biochemistry, anatomy an physiology I believe so if deakin optometry offers these subjects as part of the course then that is an option but Melbourne has to also recognise the uni's biochem/anatomy/physiology subjects as comparable to their's (I pretty sure) and so be careful when choosing a university. Personally I would think it's easier to simply do biomed or science at Melbourne as it's probably easier to meet the prerequisites and you can always studying optometry at UoM if med doesn't work out.
Many places offer post grad medicine including UQ, UoM, Monash, Deakin, UWA, Uni of Sydney and University of Wollongong and you've got a more choice in Victoria any way compared to undergraduate med.