If you get 80 internally, and it gets moderated to 80, you'll need 100 in your own HSC exam to get 90 as an HSC mark.
Your internal mark does not matter, it gets moderated based on the highest HSC mark in your cohort, and your rank and the relative positions of the people above and below you. Your "80" gets pushed up or down with the gaps between people in your cohort constant - with the pushing depending on how the top person went.
That moderated mark is averaged with your own performance in the HSC exam.
In a nutshell:
(Moderated mark based on your rank and best cohort performer + Own HSC exam mark) / 2 = Reported HSC mark