No carrier likes it's non-rev pax to be noted by the other revenue generating pax for any reasons and QF is no different. You will be treated like any other pax - fed and watered etc.
Dress code is not overly strict, but certainly there is an expectation to be dressed appropriately. No cutoff jeans, dirty clothes, thongs/flip flops, singlets etc. Dress code for Y is "neat casual" which usually means collared shirt, casual but "classy" looking comfy pants like chinos etc.

One point to remember and one that throws everyone down here, is on your return, you need to check in at a special nonrev desk and nowhere else. At SYD, look for the QF "RED S" check-in desk. That is where they process all nonrevs for all QF International flights. Won't be hard to find it - just look for the desk that has a heap of people in the immediate area sitting on suitcases and the such !!