As mentioned above, repeating year 12 is about the stupidest thing you can do. The only exception to this is if something prevented you from completing the HSC (sickness, major family/personal crisis). You are far better off going to uni and doing some courses which relate to your desired degree and then transfering (its possible to complete some units for your desired degree before you get there). This is a much better use of your time then wasting a year re-doing the HSC.
Also to add, when most people repeat the HSC, the problems that plagued them during round one also plague them during the repeat (i.e. lack of motivation etc).
Also to add, when most people repeat the HSC, the problems that plagued them during round one also plague them during the repeat (i.e. lack of motivation etc).