Uni economics units are not the same as your HSC course. High school knowledge will not net you distinctions and high distinctions because it is not detailed enough, and uni requirements are much more particular. It's been said that many students who top Eco in first year have no previous study of the subject. Working on a clean slate is not necessarily a bad thing. Besides, economics is always changing (especially labour economics) so what you learn may become obsolete. Take fundamental subjects like Maths (go ext. I or II) because a sound mathematical foundation will be be very helpful later for understanding econometrics (not just learning to number crunch SPSS).
But honestly, just choose whatever subjects you're better at and interested in. Much as I hate to say it, your aim at this stage is to achieve the highest ATAR you can. Your aspirations are likely to change in the next couple of years, so you'll want to keep your options open.