It depends on how difficult your school's trial exam is relative to the HSC exam, but to be honest it is irrelevant.
The trial exam is a key determinant of your school rank (usually around 50% of your school assessment mark). The marks your school gives you are irrelevant since they are moderated by the external exam.
Your school effectively submits a rank for each student to the BOS.
The external exam marks for everyone in your class are then added up, and distributed according to your rank. This is your moderated assessment mark.
The student who ranked 1st in your year gets the top moderated assessment mark (even if they did not actually score it). The student who ranked last gets the bottom moderated assessment mark (even if they didn't score it).
However, your HSC mark= Average (moderated assessment mark + external exam mark)
Therefore you obviously want to get a high external exam mark as possible even if you have a high school rank....