From my understanding, the hsc and trial has the possiblity of being marked diffrently( obviously depending on your school).
Also importantly to this fact, your trial mark from school will be your mark/100, but the hsc exam mark, is your raw mark(mark/100) alinged to performanve band cut offs. Therfore you could get a 82/100 in trial and get 82% for your trial, but in the hsc you could get 82/100, but be alinged to 89 ( hypotetically) due to performance band cutoffs set by markers.
Therfore,your trial mark is useful to gauge where your at, but in my opinion, it isnt a true reflection in comparision to the exam mark you will recieve when you get your results back