I think the main difference between Pre and HSC is not in the course. The only thing that actually gets harder in the HSC course is physics and math ext 2..the rest is just a expansion of what you learnt in the Pre.
But one thing is for sure as Newbie said before, people starts to compete in HSC year. In my school we have totally crap guys charging up the ranks and this can srsly disturb how you rank before in yr 11. The fact is if you go to a selective school or some very competitive school expect to get challenged since those who do crap before is smart but they don't try, now its up to the crunch time and if they try all hell break loose