It is uncommon for a student at a top 10 school to receive below 80 but it does happen. I even heard a kid at Ruse once got in the 70s.
That said they'd have to pretty low ranked in everything and not do well externally. It's arguably easier to get a higher ATAR at a top school because of the score distribution, unless you are a total bottom outlier.
But point of the story is, the HSC is attempted to be designed to be fair, if you do well individually, they will never take that away from you/you will be rewarded.