I did do year 12, but my TER was only 40! I hated school. My TER wasnt enough for uni so I had to wait until I was 20 to get in as a mature aged student.

Anyway, if you have been to TAFE they would probably look at that. Check with the uni you are intersted to see what kind of TAFE qualifications they accept. Some accept diplomas, but some also accept certificate 4. It depends on the uni and the degree you want to do. I've been to tafe but my tafe course has got nothing to do with the degree I'm doing. The uni just said as long as you can prove that you have the potetntial to be a good student then that really helps your chances. (I did IT and HR at TAFE and I'm doing Nursing at uni)

You could also do a personal statement. I got into uni this year (to a degree that requires a TER of like 83 or something) and I wrote a personal statement to get in. Basically, you just write about your reasons for wanting to do the course, any relevent experience you have (work and life) and all that sort of stuff. But your uni will give you some guidelines on what to include.
I also did STAT a lttle while ago. You can get example questions off the website of the people who run the STAT. Its all like problem solving and comprehension questions and you get given a ranking depending on how well you did compared to the other people who did the test.

Your uni will have all the info on applying as a mature aged student and you certainly do not need to have done year 12.