Yea you probably need to start going ahead in Year 10, so that you can start the HSC course for the sub in Year 11, and have it finished.
It's best to accelerate for a subject that is offered as accelerated in your school, so you have maximum resources.
The advantage of this is that say you can do 10 units in Year 12, but still have a total of 12 units (so the worst one won't count, which will be a huge bonus because the worst one usually drags your mark down quite a bit)
That said, enjoy Year 10 and 11 =)