Reiterating that above.
Either get a 280X now, or wait for the GTX970.
Looks like the 970 will sit between 15-25% better than the 280X at HD, and more at 2K+. It will also run cooler and (probably) quieter than the 280X due to the low power requirements.
I would expect the 970 to come in above your budget. USD$329 RRP, likely to be around the NZD$479+ mark after taxes.
If you are don't mind switching to the red camp, then the 280X is a solid buy, and you won't be disappointed.
As an insight: I'll probably be upgrading my GTX570's soon, and it looks as though I'll be moving to 2x970.
EDIT: OUCH. NZ$679 for a GTX970.