Start with NESA's own documents
The syllabus for your subject and past HSC papers, both freely available from NESA, are the two most reliable resources you have. The syllabus tells you exactly what can be assessed and with what verb; past papers tell you how that content has actually been tested.
Marking guidelines published alongside past papers are worth reading closely—they show the specific criteria markers use, which is often more revealing than the question itself.
Use past papers for retrieval, not just reading
A past paper sitting unopened in a folder does nothing. Attempt questions under timed conditions, mark them against the guideline honestly, and convert anything you got wrong into a flashcard or a short note on what the correct approach required.
Rotate through several years of papers rather than repeating the same one, since exam style and emphasis can shift slightly year to year.
Free tools worth combining
A spaced repetition tool for holding onto definitions, dates and data; a shared or personal document for essay plans and structures; and a simple timer for practising exam-length responses cover most of what a study system actually needs, without requiring a paid subscription to any of them.
Cueable's free tier—flashcards, FSRS-based scheduling, active recall and deadline planning—is built to cover this without needing a paywall to be useful for HSC preparation.
What to skip
Be cautious of unofficial 'leaked' papers or paid notes claiming guaranteed marks—quality is inconsistent and some content is simply wrong. Prioritise resources you can trace back to NESA, your school or a method you can verify yourself by testing it against past marking guidelines.
More resources are not automatically better. A smaller set you actually use consistently beats a large collection you never open.