Word on the street is that a selection rank is formed from 2/3 ATAR with 1/3 UMAT.
From the website:
To be competitive it is expected that applicants will require a ATAR of at least 95.00 and UMAT scores in the mid to high range.
However I would imagine that 95 would be somewhat the bare minimum considering what people have gotten in the past, and the UMAT would generally be 60-80th percentile for most people. Of course, a higher ATAR would balance a lower UMAT and vice-versa, it seems.