TwoSeven: I think the comment was a joke comment, but it does make a valid point - poorer people could end up paying more until the older vehicles are no-longer on the road.
However, I have still trying to work out why diesel passenger cars are more dangerous than petrol ones (they pay more ACC on the registration than for petrol).
Most modern diesels are cleaner than petrol and way more efficient in many cases. Sadly NZ (unlike Europe) does not recognise the difference between an ancient oil burner that looks like it is being run on coal and a new VW Passat that outputs squirrel tears (as JC would say) when working out the costs of registration etc.
UK car tax (rego) is graduated based on the amount of CO2 your car is classed as emitting - newer fuel efficient car = smaller annual bill.