I second what others have said. Either:

• Rebook straight into J by paying the fare, or
• Rebook into PE and attempt a OneUp to J (PE is quite decent for that 3 hour flight), or
• Remain in Y and attempt a OneUp from Y to PE.

Without knowing which specific flight you're on, there is always the chance that there is an aircraft downgauge on AKL-MEL and you'll then be in an all Y A321/320.