certain chemists in certain areas will do needle exchange for you, but drug and alcohol rehab and education centres (around certain hotspots and hospitals in sydney) will just give out needles, with no real need for an exchange. there are also vending machines that sell a fit-kit for a couple of dollars, which contain a few needles, a spoon, water and swabs (they work as a sharps disposal box as well, so use them).
diabetics now also get free needles, so if you can find out a valid national diabetic supply scheme number (in the format xxxxxxx-x where x is a number) or just fake one and look like a diabetic, NDSS-registered chemists will also hand them out.
oh, and don't ignore or forget to consider the parental advice offered above...