Ring the Plumbers, Gasfitters and Drainlayers Board (the Regulator) and have a chat to them - they have a licensing team who can explain the ins and outs of it all.  Bottom line is you need to satisfy them of competency - but I don't think there is anyway around not sitting the exam in your case.  While you sound like you have heaps of experience, you do need to know the NZ regulations etc and studying for the exam will help with this.  Talk to your local polytec and see if there is a study group etc you could join.  Presumably you are working on an exemption at the moment?