So I made an account just for you.
It seems everyone on here is bashing the concept of repeating year 12 and claiming that you won't improve your ATAR mark. Of course it is definitely up to the individual, but despite all the discouragement of repeating, I did so this year. I can tell you that my ATAR last year was 68.9 last year and this year it was 96.4, doing all the same subjects. I honestly don't think I put in A MASSIVE effort or had a transformation of the sought between the years but I think being familiar with the format of the HSC and examinations throughout helped me a lot.
Don't get me wrong, I still studied but repeating turned out to be the best thing for me. Now I have the ATAR for my desired course, something that I couldn't even dream of last year after my 2011 ATAR.
In the end, it's up to you
I just thought I may give you the info that IT IS POSSIBLE to improve