It really depends on the subject. They won't ask the same essay question again twice...but practice on past HSC exam questions are a must to get some sort of structure going and to really drill in some key points and ideas into your head. For an exam based on short-answer, you want to know all...
For me honestly, I prefer rewriting my notes in my own words again to re-learn the content then practice some HSC questions on that particular syllabus dot-point. Whatever works for anyone!