To work at sea, in New Zealand waters, you will require the appropriate Certificate, Ticket or Licence from Maritime New Zealand. To get this you will need to apply directly to Maritime New Zealand, with the required documents (e.g. Application form, sea time, medical certificate, approved qualification).
NMIT/IMINZ provide approved training/qualifications for many Maritime New Zealand Certificates, Tickets and Licences, however the training/qualification you receive is only part of your application to Maritime New Zealand.