this one is an odd one, but if you create an ups account, you can order packaging for free from UPS. this could be good for people trying to make international shipping etsy stores or something and need packaging(but tbh you could probably just afford buying boxes from nzpost.
seems your supposed to use them only for ups shipments, but i dont know how they would enforce that, unless they have agreements with courier companies here to enforce it.
you can only get a certain amount of each item they have in stock, so dont try to order 1000 boxes.
It's same thing with DHL as well.