Not quite true - some unis, including UNSW, do combine UMAT and predicted UAI when giving out first round interviews, which are held in the September holidays. For second round interviews, they use UMAT and real UAI. However, for a given UMAT mark, the necessary UAI to get an interview in either round is similar, but obviously if you get a much better UAI than your school predicted then you get a chance.
UNSW usually puts out something like this, which I had forwarded to me by a friend who applied there last year:
UAI and UMAT scores required for interview
Listed below are the minimum predicted UAI and actual UMAT scores that applicants needed to achieve to be offered an interview in September.
* If the sum of the 3 UMAT scores (the actual mark not the percentile rank) was 151 the predicted UAI to obtain an interview was 100.
* If the combined UMAT (as defined above) was 163 the predicted UAI to obtain an interview was 99.50.
This Table gives other cut off scores:
Actual UMAT - Predicted UAI required for offer of main round interview
151 - 100.0
163 - 99.5
173 - 99.0
188 - 98.0
194 - 97.0
205 - 96.0
212 - 95.0
Apologies for the poor formatting.
Keep in mind that UNSW, although not necessarily the best med school in Aust, is definitely one of the most applied to and most competetive to get into, so those numbers don't necessarily represent other unis but they give an idea.