The short answer to your question is 'now'.
I disagree. It's not even the end of 2012 yet. I didn't start Year 12 work until mid-January.
Take a break, people need a good one before they start studying madly over the course of next year. People will burn out so quickly if they start studying now, it's like using up all of their glycogen supply before the race has even started. (Yeah come on Biology)
The short answer to your question is 'now'.
There is a lot of work to go through VCE, some would say an inhumane amount of work
If VCE is inhumane, then university must be hell.
While Year 12 is stressful, the amount of work will not break people's backs. It's manageable if you study smart, organise your time effectively, and don't procrastinate.
It stands to reason that you will be better able to complete your preparation if you spread it over the longest time possible.
This doesn't help all the time. Sometimes, it may. But be aware that if you attempt to complete the course for subjects like Methods/Spesh/Chem/whatever in the holidays, you risk losing stimulus as the year drags on. There was a kid in my cohort who finished both Methods and Spesh courses and then became disinterested in class and proceeded to sleep and crap.
Additionally, starting practise exams early didn't really help me, I reckon. For the Chem mid-year, I finished the course by April and had 2 months to revise for the exam. In the end, some people who only started to revise for the exams 2 weeks before still did better than me.