Air New Zealand is trying to cut costs where ever possible at the moment. Kicking out some more routes, getting rid of the B1900Ds, retiring a 767 and they are about to retire some of the ATR72-500s... It is clear that they are feeling the pinch that Qantas Group is putting against Air New Zealand.
I'm sure that you would be aware that NZ is replacing the 72-600 pretty much 2-1? They have Only 11x 72-500s service, and 20x 72-600s on order to replace them, along side there 9x 72-600s already in service.
Same with the 763, they are replacing 5x 763 with 12x 789s. They already have 6x 789s in service, along with another 3x this year and the remainder other the next two years. It was always planned that the 763 would leave the fleet!
Then there is the 320CEO that being replaced by 321/320NEO from next year, and they currently have 18 slots to replace 13 A320CE)