HSC chem is significantly different from uni chem-uni chem is a lot harder in the sense you learn a lot more concepts, almost no need to write any essays/long answers-it is more about knowing the maths involved and applying skills-i.e. organic chemistry in particular with their roadmaps and stuff, as a result-HSC results are nowhere a good indicator of uni results.
One person who came 2nd in the state for chem -didn't top chem (or even came close)-and that's partially because HSC chem mostly tests rote learning skills, uni chem tests something else-in particular there is much more emphasis on lab skills.