Well I dont have time to write a long AussieChica style reply, but heres the short version:
If you want to do graduate med, your UAI becomes irrelevant as only your uni marks are used. Most unis require a minimum credit average. You also have to sit the GAMSAT exam which is like the UMAT on steriods (its not like the UMAT at all really). These factors are taken into account to decide whether or not you get an interview. Im not sure about the rest, I know some unis have the gpa as a 'hurdle' and use the gamsat and interview to determine whether or not you get a place, and some unis weighit 1/3 gpa, 1/3 gamsat and 1/3 interview.
Check out
http://members.optusnet.com.au/~pagingdr/
It has everything you want to know about grad med.
(Btw is called 'graduate' med not 'postgraduate' med. Just being nitpicky...)