You could potentially move into post grad medicine at melbourne, from any course, provided you have certain prerequisite subjects, its late so forgive me if im wrong but Biochemistry, Physiology and Anatomy at a 2nd year level. Check if the Medical imaging course provides these.
Also you'd need to sit the gamsat which is very long and grueling and theres no guarantee you will get into grad med.
You could always try looking up the course on the site or emailing them. I have, they're very helpful.
Maybe i haven't said anything you already didn't know, i suspect this is the case.
The one things i'd say to you one med hopeful to another is, really think about the radiography course. Make sure there is no doubt in your mind you want to do radiography for a long time, if you potentially get stuck with the radiography degree and cant get into grad med for one reason or another. Just really think about it, i've thought a lot about that and doing biomed, i don't know whether i would like too much being stuck with a biomed degree i don't want. Maybe compare the opportunities you get in biomed vs medical imaging