An Inland Revenue Department (IRD) number is the tax identification number that you use for your tax-related activities within New Zealand.
You can apply for your IRD number online, but you will need to have your identification documents verified in person.
If you are coming to New Zealand you can apply for an IRD number when you arrive:
Apply for an IRD number — living in New Zealand and not a new arrival
Apply for an IRD number — new arrival to New Zealand