Dates, dates, dates.
Christmas is probably the busiest time on the routes from here to Australia and New Zealand and the airlines know it and take advantage (and I don't blame them). You would probably need to go no later than mid December and/or come back after mid January to avoid the worst fares although, as someone has already said, the slot between Christmas and New Year may offer some relief. Also think about New Zealanders and their holidays. I've just booked to go out on 5th January to Hong Kong on Air NZ continuing to Auckland on the 10th. I wanted the 11th but the class I wanted was already full which I reckon is about New Zealanders going home.
My view is always that the cheapest fare is unattainable. You need to find something you can live with and then book it. Leave it and you won't get there.