When sending SMS messages, the safest option is to send numbers in internationalised E.164 format, e.g. +642100001 and +61400010001.

Your account can be pre-configured to accept New Zealand and Australian formatting. This means you can submit NZ and AU numbers as a mixture of:
+6421000001 and +61400010001
006421000001 and 0061400010001
6421000001 and 61400010001
021000001 and 0400010001
21000001 and 400010001

The service is able to accept and strip any unnecessary characters, such as spaces, dashes and brackets.

For Voice and Fax messaging, see: Mobile Number Formatting for Voice & Fax