According to http://www.nzta.govt.nz/resources/factsheets/71/registration-plates.html
Black & White plates will only be issued to cars, that are been re-registered, that last had a black & white plate, and only if it's the same plate number (i.e. that car you had with the plate number AA1234, if you want to re-register it, you can apply for black & white plates, but only if before deregistration it had B&W plate).
Other than that, looks like the owner would risk a (up to) $1000 fine.
Excerpts from the factsheet:
Older style plates, ie the black plates with silver characters, can only be duplicated with a white retro-reflective background and black characters
In certain circumstances the NZTA will allow the re-issue of a vehicle's older style black and silver plates. The following conditions apply.
The number on the plates must be still available. Sometimes people use a number from the old black plates and put it on a personalised plate ? in such cases the old plates cannot be re-used as the number has been 'taken'.
The plates must be in good condition.
The plates must be the last plates that were issued to the vehicle when the registration was cancelled and the owner may be required to show evidence of this. The best evidence is the certificate of registration or other paperwork (insurance policy, repair receipts showing the registration number, etc).
This rule only applies to those who are re-registering their vehicle. For example, if a vehicle is currently registered with plates but previously had black and silver plates, these cannot be re-issued.