Past papers are an excellent source of practice and there are hundreds of them around. Just remember that the ones prior to last year had a different syllabus so there are some questions that are no longer relevant (not that many, but some) and there are some topics not covered. The only type of practice that is limited is past HSC papers, so starting now does not mean running out of practice. However, you need to be selective about which questions to do to make sure you are covering your current topics. This becomes easier once you have finished the syllabus.