Hi there able58. Good luck with that one! If you work it out, please let me know!
However, bookmark http://grabaseat.co.nz/ and subscribe to AirNZ offers - that way when they drop the prices for Auck - Bay of Islands(Kerikeri) you'll get a heads up. An alternative is to fly with Flight Hauraki (ex Saltair) http://www.flighthauraki.co.nz/saltair/ who pick up from Auckland CBD then fly out of North Shore to Kerikeri for consisstently reasonalbe prices - good value if you are already in Auckland central, and at least you know you're not paying AirNZ's outrageous full fares - more than Oz to NZ for a 1/2 hour flight.
Hope this helps.
Edited: 3:29 pm, December 31, 2012