There is an issue with the address auto-suggestion system that is used by Shopify for New Zealand addresses.
When an address is chosen from the auto-suggest, the suburb of the address will be placed into Address Line 2, which is labelled Apartment, suite, etc. (optional). This label is incorrect.
For example, if we typed 1 Albert Street into the Address field on the checkout and selected it, it would populate the fields as below:
We need to change the label on Address Line 2 from Apartment, suite, etc. (optional) to Address Line 2.
Login to your Shopify backend and on the left side navigate to Settings -> Checkout.
At the bottom of the page, click "Manage checkout language":
Under the Checkout contact section, modify the field "Optional address2 label" to be Address Line 2.
Scroll back to the top of the screen and press Save.
Your checkout will now look like the below and auto-suggest will behave correctly.