I disagree with sbiddle. When you buy a business class fare you should be given business class, not economy. If NZ doesn't have business on the A320 (an economic decision) then they should put you on an airline that does. If you went to Toyota and paid for a new Land Cruiser and we're given a Yaris would you be satisfied?
I have been very close to this situation twice in the last 6 months and barely made the plane, there was no help and they really don't seem to care.