I assume you mean practice papers?
For practice, its never too early to start, especially for an accelerated student. Obviously the syllabus relating to the School Certificate must be covered in order to do the examinations. But in just about every case, the questions asked are no more than common sense and basic skills.
I'd say English-Literacy, Mathematics, Science and Computing Skills (from next year) can be practiced right now. I was given English-Literacy practice papers to do as an assessment in Year 9 anyway.
As for Australian History, Geography Civics and Citizenship, the main topics tested (especially for Australian History) change every year or every two years. The topics in the syllabus must be covered completely in order to practice for this exam, since the examination (especially short-long responses) will be likely to ask things that should be kept on memory and specific topics or research done in the classroom. For example in Australian History, you might be tested on a sight study of the heritage of specific site you may have researched in class & in Australian Geography you might be tested on a fieldwork assignment you may have conducted during class.
In general, you can practice for any School Certificate examination other than Australian History Geography Civics and Citizenship, since classwork/research can be a potential examination question (or questions) in the Australian History Geography Civics & Citizenship examination. Most students are only truly prepared on the day of the examinations anyway.
Plus for an accelerated student, you seem like you're really stressing a lot as if it were the Higher School Certificate. Don't worry you'll do just fine. If I can get 3 x band 6 and 2 x band 5 in the 2004 School Certificate without much study so can you, if not better.