Bah, I skimmed over it and I missed the out behind with. It's one word dammit.
Trials are typically worth anything from 10-40% of your trial mark. Depends all on the weighting of the trials. Anything <30% weight doesn't have too great an impact on your final assessment mark, but it kind of sucks out all your enthusiam or passion for the subject which is the main concern.
60% of 30% would mean that you'd lose 12% of your total assessment mark, which if you take it into perspective, is only worth 6% of your HSC mark.
Your biggest concern should be about the actual HSC exam.