I know that at the Sydney College of Art they'll look at your portfolio and then you go through an interview process. As a general rule you need to have a UAI over 70 to have a real chance of getting in (their website says so).
For the National Art School, entry is also by portfolio, interview, and on the apllication there's a questionnaire you have to fill out. They don't look at your HSC mark at all - it's all based on your talent (which should be evident in your portfolio) and creativity (that's what the interview is for).
CoFA will select you purely on your UAI, so as long as you reach the cut-off then you're in.
What I know about those colleges comes from what I've read on the websites and what others have told me. The 3 that I've mentioned are all really good, but I guess it depends on what you want to get out of it.
The National Art school is a much more traditional art college and places a strong emphasis on drawing (which is compulsory throughout the duration of the 3 year course).
CoFA gives you much more freedom and helps develop your own style. My teachers who went there told me that you don't have anyone telling you how to paint or draw, it's something you develop yourself whilst in the course, this way everyone's style ends up different.
I don't know much about SCA but I know they offer lots of different electives, and my teacher said they have a great performing arts faculty.
Hope I was of some help
