Overbooking, rather than shuffling things around due to a disrupt must be extremely rare on NZ. I have flown them since 2009 and have been elite since then and have flown NZ about 1,400 times (according to my Flight memory account). I have never heard an announcement at the gate where they have looked for volunteers to offload. Nor have I been approached at check in to ask if I wished to volunteer. I think my rate for hearing the overbooking announcement on my US flights is perhaps 25%, in comparison.