It is rare but from time to time NZ have been overbooked they have asked for volunteers to catch a later flight. Usually some amount of money (NZD$400) and accomodation and maybe /C/ on the next flight. Depends on the situation and the out-station.

Whenever I have seen it offered it has been rare (NZ do not overbook like say UA might), and they usually have no problem finding people.

Mark