royaaran, the scheme you are reffering to is the RET which uses STAT results to produce an alternative entrance rank into UWS only.
Other uses include mature age entry(and this is what the UAC site catalogues when they say whether unis accept STAT), it also used as an additional criterion for USYD BPharmacy.
If you are interested in the RET, you only have to wait as UWS will mail out a package to all eligible candidates when the time comes. If you are interested in the STAT for Pharmacy however, you may take either the upcoming session(which is the same one as the RET people do) or the one in I think November.
I myself did the RET/STAT, and while I didn't end up choosing to go to UWS I believe it was well worth taking, if only for the opportunities it opened - after taking it, it gave me a score that would have allowed me to pick any course they offered. And considering it also helps meet the criteria for certain other courses, as well as giving you an alternative entry method for you to use when you're mature age then it was most certainly worth it.
It really isn't that hard - you don't need to study for it or anything. Its more a general aptitude test than anything, requires logic and little more. It is divided into two pieces, firstly the multiple choice which has a series of qualitative and quantitative questions, and secondly the essay component where we had to write two short essays on given topics. If you're interested in Pharmacy, you won't need to do the essay bit though it is required for the RET scheme.