I have never been to NZ so you have me at a disadvantage there. But surely a system registering suitable vehicles would be better. From what you and others have stated these issues involve cars or vans that have been “converted” for use. Once you have carried out any conversion it would be inspected at the cost of the owner before being allowed to WC. If you are caught without an appropriate vehicle then you would be heavily fined or for second offenders your vehicle would be impounded. To simply ban people who have spent a fortune on proper camper vans, and who have behaved properly seems draconian.
But what I don't understand here is there is no mention of tents. It appears from what I am reading that people would still be able to WC using tents. Up here in Scotland most of our problems come from tent users who simply don’t give a dam. I can see those with these vehicles simply migrating to tents, and behaving as before, whilst those who have properly converted camper vans being banned.