Hi Andre and
I'm not sure how you'd go about becoming a plumber in NZ because a plumber is required by law to be registered and it may affect your application if you haven't got recognised qualifications. This is the official
site for plumbers, gasfitters and drainlayers.
I'm sure you would have to complete some sort of examination before becoming registered. The vital element here is whether they would accept you with no formal qualifications although I believe the PDGDB isn't totally accepting of quals from outside the country anyway. There's quite a lot of blurb on
here , but have a read through and see if it answers some of your questions about the process.
It could be possible that you'd have to retrain in NZ and then go for registration. As plumbers are much needed there, you may be lucky to find an employer who will put you through the training.