(As the OP has expressed concern about no visa-waiver specific guidance and wanting advice specifically from the Immigration New Zealand website, I have edited my question to make this exceptionally clear)
The August 2017 Visitor Visa Guide INZ 1018 has this to say on page 3, as part of the "Visa-Waiver visitors" section (starting on page 2):
If you are in one of these groups, you are a ‘visawaiver
visitor’, and you do not need a visitor visa
to travel to New Zealand. You need to apply for
a visitor visa, but you will do this by completing
an arrival card on the aircraft/ship on your way
to New Zealand. You must also:
- hold a valid ticket out of New Zealand to a
country you have the right of entry to
- have enough money to support yourself during
- hold a passport valid for three months beyond
the date you are leaving New Zealand.
Emphasis my own.
This is backed up in general by other New Zealand government websites giving the same advice across the board.
According to the official New Zealand tourism website, your passport has to be valid for 3 months after your departure date:
When you arrive, you’ll need to ensure your passport is valid for at least three months beyond your intended departure date, and if required, have a valid New Zealand visa.
This is supported by the NZ Customs website:
To enter the country you must have a passport and it must be valid for at least three months past the date that you intend to leave New Zealand.
On the main NZ gov site:
a passport that's valid for at least 3 months after the date you plan to leave NZ