As is usually the case, Air New Zealand has its best deals on its own web site. (In fact, they now have a fare guarantee where they'll match any lower price you might find elsewhere, plus a $25 bonus).
Until May 31, they're offering some sweet deals to New Zealand with a free stopover. For example, we found Los Angeles to Fiji to Auckland for $951 including all taxes and surcharges for travel mid May to early June. ANZ's site is an absolute snap to use, clearly showing you exactly when the cheap seats are available for a given sale. Kudos to those clever Kiwis.
Some rules for you to mull over:
Travel July 21-September 7 is higher, but still good value.