Yeah the HSC is the easiest and first way to get into uni. Thus, why not take the first opportunity?
You may find that some courses in uni require you to do chemistry-incase you may change your decision of to what you want to do in uni. I dont think i've come across a course whose prerequisite is physics/biology. My 2 cents is I think you should keep your options open and pick a science pref, chem. It is a hard and demandind subject, but if you pay attention and keen to work, then things will ship ahead for you.