I always suggest to leave the HSC papers for the final HSC exam.
Whilst some HSC questions are reused (or reworded) in the trial exams of certain schools, for the most part I prefer leaving them untouched. This is because despite the massive similarity, you want to leave them for the actual HSC exam. By doing them now, you may feel inclined to skip questions here and there later on the excuse that "oh, I've already done these questions already so I can't be stuffed".
This of course implies (but is not implied by) that for the trial block, you should be doing trial papers off past schools. If you can land your hands on CSSA or Independents papers (I'll assume you're buying them or your school has retained copies because we don't want to do illegal business here) then that's brilliant. That being said, many non-copyrighted papers are available on sources such as
THSC and you simply cannot run out of material to use.
(For harder questions, I recommend the Sydney Grammar and Knox questions.)