In order to use New Zealand Post, you'll need to ensure you are using a valid New Zealand address as your store location. You'll also need to ensure your Craft Commerce default currency is set to NZD.
New Zealand Post doesn't offer a set list of services for you to enable or disable as required. Services are automatically returned based on the matching criteria with your shipping origin and destination.
Add the following code to your configuration file under the
providers array, as per the below. Note that to disable certain services, simply omit them from the
'providers' => [ 'newZealandPost' => [ 'name' => 'New Zealand Post', ], ]
Available for Craft 3. Get it from the plugin store.