Let's talk about bans when there is suitable and affordable technology to replace even compact petrol cars in all use cases. That isn't yet the case.
Currently small to medium EVs that would suit most households cost more (to buy) and have less capability than equivalent ICEVs. Once you get into higher end cars like Tesla S, I actually think EVs are better value, but fewer people can afford them.
Organisations' fleets seem a better case for early uptake, depreciation or finance costs are tax deductible and there are a lot of short trip use cases - council's DHBs etc. They would respond rapidly to incentives.
You also need to be really sure the supply will be there. Supply of new EVs to the NZ market has been patchy at best.