From Vodafone's website (www.vodafone.co.nz):

Prepay outage

Some Prepay users may be experiencing issues with making voice calls and sending TXT messages. This is only impacting some of our Prepay customers. We are working on this issue at the highest priority. We apologise for any inconvenience. We will put updates here as we have more information. You can subscribe to the answer to be notified.