As I have said previously one of the problems of reducing the number of MPs to 100 is that over time we will have less and less List MPs, almost guaranteeing overhang situations where we lose proportionality.
Basically so long as the North Island grows faster than the South Island (SI seats are set at 16) then the number of electorate seats will grow. This happens every five years after each census.
Now I’ve gone to the official Stats NZ projections from the 2001 census. They have projections up to 2026 for NI and SI populations and projections up until 2016 for NI and SI Maori populations (which affects the Maori seats).
Now this has allowed me to do some projections as to how many electorate seats there would be (based on 2001 census) after each census. Using medium growth assumptions it is:
If however the South Island followed a low growth path and the NI high growth then the no of seats would be:
This has been based on 60% of Maori choosing to go on the Maori roll. If this percentage increases then the numbers grow slightly more.
So even if one did decrease the number of seats to 100, you would almost be forced to increase it again in the next 10 to 20 years.