Have you got the ones on here? And go to VCAA for official past exams (but you probably already have).
Hope it helps! 
(btw isn't it a bit early for prac exams? -not that I did many
)
I did my first practice exam today. fml got about 50%
Its a bit early to do practice exams. You will run out papers to do by the exam period. I highly suggest that you take more time into revising concepts and have a wider reading of the material rather than doing past papers.
This is because you may not fully understand the questions, hence your 50%, and then when you redo the question you will already know the answer.
There are various e-books and three different biology textbooks (Nelson, Heinamenn and Nature of Biology) which you should read.
Last year I owned all three books and found that each one explain something differently which made my understanding of the fundamentals much clearer.