Under MMP, major parties winning electorate seats is largely a political sideshow: winning or not winning them has never changed the total number of seats allocated to any party above the 5% threshold.
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Get the latest headlines straight to your inbox.Every five years - when new census results are available - the Representation Commission reviews New Zealand’s electoral boundaries.
This means boundaries remain unchanged for at least two elections between reviews.
The Commission’s priority is to make sure Electoral Districts are all roughly the same size. But the different populations in different regions of New Zealand don’t make that process easy. Skip the next six paragraphs if you don’t like maths.
The Electoral Act defines how the calculation works. It starts from the somewhat arbitrary requirement that the South Island always has 16 general electorates. The ideal electorate size - 70,037 - is defined by dividing the South Island general electoral population of the South Island divided by 16.
Then it gets more complex.
The number of North Island electorates is defined as the closest actual number of complete electorates to the North Island general electoral population divided by the ideal electorate size.
So 3,353,982 North Island voters divided by 70,037 is 47.88. But you can’t have 47.88 electorates so the North Island will have 48 electorates.
This means the ideal North Island electorate size is 3,353,982 divided by 48, or 69,875.
A similar process is used to determine the number and size of the Māori electorates.
The three ideal electorate sizes are then used to calculate a minimum allowed, and a maximum allowed population size for each of the three types of electorate: Māori, South Island General, and North Island General.
The Representation Commission - with an eye also on future population projections - then adjusts electorate boundaries to ensure that the population in each electorate is between the minimum and maximum allowed size.
The new review has proposed to remove one electorate, and change the boundaries of 49 of the remaining 71 electorates.
The seven Māori electorates are relatively unchanged. Changes are proposed for only two Māori electorates, and the numbers of people affected will be less than the winning margins in both those electorates.
Using the simple assumption that voting is uniform across an electorate, we can estimate the impact of the boundary changes on the 64 general electorates.
In 2023, Megan Woods won the Wigram electorate by 1179 votes. The new Wigram electorate will consist of 11,366 people from the former Selwyn electorate and 64,414 from the former Wigram electorate.
Selwyn is a National strong hold. In 2023, Nicola Grigg won with National’s third-largest margin.
In the new Wigram electorate, if those 11,366 former Selwyn voters reflected the average Selwyn voter, National’s Tracy Summerfield would have defeated Megan Woods by 2554 votes.
If you think the area proposed to move from Selwyn to Wigram doesn’t have the same political leaning as the rest of Selwyn, or that individual candidates would have swung voting preferences, you can use the interactive sliders above to adjust the result.
The new Rānui electorate will be made up of 39,955 voters from the former Te Atatū elecotorate, 18,431 from Upper Harbour, 4566 from Kaipara ki Mahurangi, and 4088 from Kelston.
In 2023, Labour’s Phil Twyford defeated National’s Angee Nicholas by just 131. But in Upper Harbour, National’s Cameron Brewer won comfortably. In the new Rānui electorate if the almost 20,000 former Upper Harbour voters voted like Upper Harbour as a whole, Angee Nicholas would have won comfortably and Phil Twyford’s list position of 49 would not have returned him to parliament.
The Mt Albert electorate was even closer. Labour’s Helen White defeated National’s Melissa Lee by just 18 votes.
The proposed new Mt Albert electorate would comprise 45,706 Mt Albert voters, 14,294 from Mt Roskill, 3736 from Kelston, 2276 from Epsom, and 1024 from Auckland Central.
Computing the contributions from each of these electorates would have given Melissa Lee a lead of 354.
The proposed new Waitākere electorate will be made up of 43,422 voters from the former New Lynn electorate and 23,460 voters from the Kelston electorate.
National’s Paulo Garcia defeated Deborah Russell by 1013 votes. But in neighbouring Kelston Labour’s Deputy Leader Carmel Sepuloni won by 4396 votes.
If we assume the 23,460 voters from the Kelston electorate would have voted like the rest of the electorate, then Deborah Russell would have won by 991 votes.
The Herald analysis was based on the blunt assumption political preferences are uniform across an electorate, and individual candidates don’t impact voter choices. Use the interactive below to apply your own local knowledge and political assumptions to see how the boundary changes might play out across any general electorate.
Submissions on the proposed boundaries are open until April 27.
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