The above is all good advice, imho. It depends on your reasons for wanting to repeat. Is it because you're not going to get into the course you want with your UAI? Or, as Bon said, are you just unhappy with your marks? Or something else?
Personally, neither of these would induce me to repeat year 12, because there are less stressful pathways (tafe and uni transfer) which will also probably give you a wider range of experiences and facilitate you down life's path. Remember to get advice for the best courses to transfer from, as Sarah said.
If you are feeling down about results, remember that after the first 6 weeks of uni or tafe, they become completely meaningless, unless you are transferring, and the extreme media hype which is the HSC at the moment will die down soon. I have a friend who got a UAI of .05 above what she needed to get into Arts at Sydney uni when she did her HSC two years ago, and now she is enrolled in Media Communications at UTS. She told me that after being really disappointed in her HSC marks and UAI, she thought she'd never get over it, but getting into university made her realise that it was time for her to move on with her life and enjoy it for what it was, not what it could have been. Now she's doing what she wants, and it took her years after her hSC to finally work it out!
Always think forward.