I've been talking to Amazon about their shipping. This answer may interest you...
Unfortunately, there is no way to request a specific shipper.
We currently use IMEX, DPGM / DHL Global Mail, I-Parcel, Aramex, MSI, and DPGM_TNT for international Standard or Expedited shipments. For Priority international shipments we use UPS International or DHL International Priority.
They are aware of issues with NZ deliveries and are working with NZ Post on this.