Mains voltage
The mains voltage (230 Volts) in New Zealand is well regulated too, and may fluctuate only a couple of volts either way.
In winter when load controls are applied (Hotwater) perhaps for a short period the voltage may be up or down 5 or 6 volts at the time to give the tapchanger in the substations to catch up.
As already said by our other members,these lamps fail because of
High voltage
Heat trapped in fitting (insulation batts put over fittings)
Fingermarks on bulb
Vibration
Arcing contacts (especially in cheap fittings)
Lower voltage, (10 to 20% under), if lower the filament doesn't dissipate as much heat and the heat stess isn't as bad. Similar like the old valve tubes.
Nigel, how did you work out he is from NZ ? The fields are not filled in, or did you see his email address.