To briefly explain what UCAT is, it is a 2-hour exam in which certain universities across Australia (also NZ, UK and Ireland) take as a compulsory requirement for school leaver (provisional) entry into medicine. It has 5 sections: verbal reasoning (English comprehension), decision making (weird logic problems), quantitative reasoning (Maths), abstract reasoning (patterns) and situational judgement (actual decision-making based on a situation).
As for the universities, I believe only Monash University requires UCAT for school-leaver entry into medicine in Victoria. The other medical degree is from the University of Melbourne which doesn't require UCAT but requires a very high ATAR (i.e. 99.90, I believe). To answer your other question, you DO NOT require UCAT to gain entry into just the general undergraduate (UNG) science or biomedical science degree. You only require UCAT for provisional entry medicine where you complete a UNG degree first (e.g. biomedical science or medical science or even science) and as long as you maintain a GPA of 5.0 (this could change at different unis), you gain direct entry into the medicine program (i.e. Doctor of Medicine). But don't worry, on your VTAC application, the general UNG science degrees and the medical degree (where you do the UNG degree first) are distinguishable. If you would like to do the general UNG science degrees, just put that as is on your VTAC application. However, if you want the medicine course (where you do the UNG degree first), it will be stated as medicine on your VTAC application.
Anyways, sorry this is so long but hope that helps. Feel free to ask any more questions!