To work in New Zealand you will need a work visa. There are a range of visas available depending on where you are from.
Once you're here you'll also need to obtain an IRD number (a unique tax number) and a bank account into which you can be paid.
If you are planning on WWOOFing (volunteering in exchange for accommodation and food) you will not need an IRD number or a bank account but New Zealand law still requires you to hold a work visa.
For more information on work visas and working holiday programmes please visit the New Zealand Immigration Service’s website:
A critical part of a Working Holiday Visa is your New Zealand tax status. Read up on what you need to know about New Zealand tax and getting a tax refund.