If you want to employ a contractor a New Zealand to work on your residential or commercial property, you will almost certainly need to employ a registered contractor. This is for safety and insurance reasons and also for the long-term warranty of the finished job.
If you want to employ a roofing contractor for example, then you will need to choose a licensed building practitioner or LBP if you are wanting to build a new roof. Although this is not legally required for roof repairs it is a very sensible approach, for long-term warranty and insurance reasons and also for work quality reasons. A licensed building practitioner has to be re licence every year which requires attending training courses and additional study.
In any case your contractor should also be a registered plumbing contractor with the appropriate plumbing qualifications or NZQA roofing national certificates. There are seven such certificates.
You may be tempted to choose a non registered or non qualified plumber because their price might be a lot better than other plumbing contractors. This approach is probably ok as long as you are very closely supervising the job and as long as you fully understand what the job entails and what a good finished job should look like. Get your plumber Wellington quickly before someone else does.
You might use a unregistered or non qualified roofer four situations where you need someone a job to climb over the roof and do small jobs which might be not comfortable for yourself. You also might use an unqualified roofer as an assistant, but you will need to be very careful that you have fully explained all the safety requirements when waking up on the roof. Remember that anything that happens on the roof if you are supervising the job is your responsibility entirely, and you will be liable for any accidents that happened on site.