i have knowen a few people in other trades that have done the same and as far as i know they the employer have to fill out some forums for immigration in conjunction with your work permit application basicly offering you full time employment etc etc...and all should be ok...i have a friend that did this and is working at bakehouse as a trainee baker and another that is a steel rigger in the construstion area...
Get in contact with NZ immigration and ask how the whole thing works they are pretty helpful as far as i have had dealing with them.
good luck