Unfortunately there is no off-season downunder. Even though it is their winter during our summer, the airlines recognize this is also the time most NAmericans are likely to visit that part of the world. Thus no off-season. The best fares are generally available early in the new year. The family Chirstimas travellers have done their trips, and most NAmericans don't want to travel the long and far for a week-long winter break. This means there's enough excess capacity to drop fares. Same goes for late November/early December.

For the best fares, check out the QF and NZ websites for great deals ex-LAX or SFO. Last year, QF was offering flights for under US$500! NZ had some good fares earlier this spring ex-YVR too.