When sending Voice and Fax messages, the safest option is to send numbers in internationalised E.164 format, e.g. +6495005000 and +61200010001.
Your account can be pre-configured to accept New Zealand formatting. This means you can submit NZ numbers as a mixture of:
The service is able to accept and strip any unnecessary characters, such as spaces, dashes and brackets.
For SMS messaging, see: Mobile Number Formatting for SMS