My BF did RET a few years ago to get into UWS, from what I remember RET is only accepted if you are not eligable for a UAI, that is if you don't meet the Board of Studies requirements for a UAI by not taking the correct combination of subjects.
RET can not be used if you got a UAI but it was crap. If you get a crap UAI then you will need to go to TAFE or something and do well and apply.
The RET exam is exactly the same as the STAT exam and heaps of uni's accept STAT for entry. There are restrictions on both RET and STAT.
Your best option to find the information you require is to contact the university.