Cellphone numbers in New Zealand dialling Australia

I am hoping this query is a simple preference I have missed, but I keep having issues with mobile (cell) numbers entered into Apple Contacts dialling Australia when used in Apple Messages.

I am based in New Zealand where the two main mobile carriers have phone numbers with prefixes 021, and 027. When a user calls me on my iPhone that is the number as displayed (eg 021 999 999). If I select Add new contact then that number format stays.

If I try and send an SMS message to that contact with Apple Messages it assumes that the contact is in Australia and appends the country code +61 to the number, therefore sending the message to +61 2 199 9999, which invariably fails.

New Zealand's country code is 64, so if I edit the Contact card to make the number +64 21 999 999 it is fine, but that is a pain to have to do that every time I add a new contact.

I am currently running Mac 10.13.1, Contacts is on v11, and my phone is on iOs 10.3.3.

In the Preferences for Contacts I have the Address Format set as New Zealand, and I cannot see any setting in Messages to interpret numbers as the local (NZ) format.

Any thoughts?