AUD 350 is about right. Competition across the Tasman is much greater than to CNS. I would not burn my AA miles as the AUD is in the toilet and 350 converts pretty well.
If you fly QF don't forget that you get lounge access in CNS though I think you may be pushing it for you and four guests. That said, I'd give it a go.
About AUD200 each way on QF is about right. Specials seem to crop up every few months so just check the QF site periodically.
If you have never been to Sydney before then it is worth spending the day but I would do this on the way home. If you have already traveled 20 hours then a few more up to CNS won't matter. If you are coming across on QF then transfers are easy but still a pain in SYD. If flying UA then the pain and time is greater.