Just because you and I are both using a litre of petrol does not mean we are polluting the same amount. Emissions testing is more fair, those who pollute pay for it.
Actually as far as the topic of this thread goes "Climate Change Policy" it pretty much does mean you're polluting the same amount.
From the figures I read, average fuel economy of vehicles first registered was improving, but since about 2012 has remained fairly static, the average age of the NZ vehicle fleet has increased. The average miles per annum per registered vehicle has also increased.