ill give a simplified answer to wat Laz would give...
since your coming 1st with scores of 90%, it looks like u'll do well in the HSC...say if u get 95 in the HSC exam, since your ranked 1st, that exam mark becomes your moderated assessment mark, therefore ur result would be:
Moderated Assessment mark: 95
Examination mark: 95
HSC mark: 95
the rest of your class wont do as well, but for your own scores, as long as u are 1st, the rest of ur class wont matter provided u do as well as u are now, in the HSC exam