Are you talking about a moderated assessment mark of 80? If so, and you get 98 say in the externals, your HSC mark will be (80+98)/2=89 -> which won't be a state rank but you'll have a state rank equivalent external. But honestly, if you're getting 80 at school, there is no way you are somehow getting a state rank -> a state ranker would be ripping the sub even without going hard out.
But yes, the externals have a big effect.