Wide bodies are often designed with lower cycle counts. The narrow bodies usually are designed for more cycles.
The wide bodies would take too long board and deplane compared to flight time.
Most domestic gates in NZ are designed for narrow bodies. WLG has some swing gates which are widebody capable on North pier but AirNZ does have a domestic longue in North pier.
Wide bodies have a lot of mass to carry fuel. Mass requires fuel to carry around for no benefit. Which is why on medium haul the A330 can beat the 787s for fuel economy as lower empty mass.
Also if switched to wide-bodies would drop the convenience hourly flights and maybe push out to two or three between flights which would be awful. It's one of the benefits of the ATR72s WLG CHC as they can keep the frequency hourly.
AirNZ will over next few years boost capacity with the domestic A321s.