It would be interesting to know how you got on as I flew to NZ from WA Australia about 18 months ago and the whole bike box routine was a nightmare.
A large Australia carrier refused to sell me bike box without producing a ticket for the flight.
I wanted to buy the box first due to not being sure my touring bike would fit in their box which is not what you want to find out on the day.
I would now fly via any other airline.
Air NZ didn't provide boxes from WA so had to buy a "standard" box from a "packaging firm".
Turned out it was outside Air NZ dimensions but an easy fix with a carpet cutter and a ton of tape.
All ended well in the end but rather than go through all that again, I've bought a Groundeffect Tardis
https://www.groundeffect.co.nz/collections/bike-bags/products/tardis-compact-bike-bag

PS. Be aware if you have a large framed 29er, I was recommended to get a newer version of the bag as the earlier version was smaller and not a comfortable fit.

 

Honestly taking mountain bikes to NZ was the easiest bike travel I've ever had. I picked up some cardboard bike boxes from my LBS and packed up our bikes at home, checking their weights on a scale to make sure we stayed below the weight limits. Then we called a van cab to take us to the airport, in order to fit our bike boxes.

Once we got to NZ we just unpacked our bikes, assembled them, shoved the cardboard boxes in a corner, threw our gear into our backpacks, and rode off to our airbnb.

Getting around NZ with a bicycle (for when the distances are too great to cover BY bicycle) is a breeze. The NZ equivalent of Greyhound runs the whole perimeter and accepts bikes, though you do need to remove the wheels, pedals, and ask really nicely. Taking the bus is awesome because you can hop from MTB destination to destination without needing to learn to drive on the wrong side of the road, and you can nap along the way. And not needing to lug a bike box around with you means when you get to the bus stop, you can just hop back on your bike and ride to your lodging.

When we finally made it back to the airport a few weeks later we just handed over $50 for some fresh bike boxes and packed our bikes back up for the loooooong flight home.

 

Where did you buy it in the airport?

Honestly taking mountain bikes to NZ was the easiest bike travel I've ever had. I picked up some cardboard bike boxes from my LBS and packed up our bikes at home, checking their weights on a scale to make sure we stayed below the weight limits. Then we called a van cab to take us to the airport, in order to fit our bike boxes.

Once we got to NZ we just unpacked our bikes, assembled them, shoved the cardboard boxes in a corner, threw our gear into our backpacks, and rode off to our airbnb.

Getting around NZ with a bicycle (for when the distances are too great to cover BY bicycle) is a breeze. The NZ equivalent of Greyhound runs the whole perimeter and accepts bikes, though you do need to remove the wheels, pedals, and ask really nicely. Taking the bus is awesome because you can hop from MTB destination to destination without needing to learn to drive on the wrong side of the road, and you can nap along the way. And not needing to lug a bike box around with you means when you get to the bus stop, you can just hop back on your bike and ride to your lodging.

When we finally made it back to the airport a few weeks later we just handed over $50 for some fresh bike boxes and packed our bikes back up for the loooooong flight home.

 

Make sure your bike is clean, with no muddy tires. NZ takes biosecurity seriously. We didn't bring bikes but I think we had to declare our hiking boots and certify that they were clean.

 

Yup! Did the most thorough bike cleaning of my life before packing the bikes.

And we had to throw away all our granola bars

 

Cheers for the prompt response OP