Exploitation of the overhang loophole has always been possible, but no party has tried it because of how voters are likely to react.

Electoral systems don’t only have rules, they also have norms: general understandings that encourage fairness between election participants. With its recent law changes banning voter enrolment during the advance voting period, the government has undermined one norm, but there’s now a suggestion that Te Pāti Māori may try to undermine another by attempting to engineer an overhang to undermine the proportionality of the MMP voting system. That’s more serious.

If there’s one thing most New Zealanders know about MMP it’s that MMP is a proportional voting system: the party vote matters most and as long as a party crosses one of the thresholds (5% of the party vote, or one electorate) then that party’s share of the seats in parliament is going to line up with its share of the party vote.

That’s a pretty big reason why New Zealanders voted to ditch the first past the post system in favour of MMP. They had seen an electoral system where the share of votes a party got seemed unrelated to how many MPs that party got, and they wanted change. But the New Zealanders who designed MMP, campaigned for MMP and voted for MMP (twice) saw some benefit in people being able to elect a local MP as well, which is why we didn’t end up with an internationally more common list-only system. We have a system where people vote for a local MP and then also a party via their party vote, with the party vote being more important in determining the overall result.

One consequence of this mixed system is that sometimes a party can have one or more MPs with very strong local support without the party having much support nationwide. A party can have more local MPs elected than their share of the overall party vote would suggest is fair. This creates an overhang, with the size of parliament increasing to ensure that all the other parties get their fair share of seats. It happened in 2014, when United Future MP Peter Dunne won Ōhāriu, while his party got only 0.22% of the party vote. And it’s happened several times with Te Pāti Māori doing well across the Māori seats, while not getting a high enough party vote to account for all of the individual seats it won.

But while an overhang has been created several times, two things have never happened: An overhang hasn’t changed which party got to govern, and no-one has tried to engineer an artificial overhang in order to deliberately cause a disproportionate result.

There have long been devoted party stalwarts who have argued that their political party should try to deliberately engineer an overhang by separating the MPs who can run in safe seats from the party that would contest the party vote in order to over-inflate the share of seats in parliament, but these proposals have always been rejected because MPs know voters would punish such shenanigans. Now, for the first time at this election, Te Pāti Māori appear to be indicating they may use the Māori seats to try this strategy: telling their supporters to only use their electorate votes to support the party’s candidates, while suggesting they use their party vote to support another party.

If Te Pāti Māori succeeds in using the Māori seats to undermine the proportionality of parliament it may work in the party’s favour for a term or two, but it also creates risks to the long-term public acceptance of the Māori seats.

International experience is that when one party undermines political norms, other parties will escalate in response. This has happened in elections in Lesotho, Albania, and Italy, where one party’s attempt to subvert the electoral system, saw others respond in kind. In New Zealand, this could mean a couple of things: National deciding that it’s now willing to go along with proposals from New Zealand First and ACT to get rid of the Māori seats, or parties on the right reacting by trying the same thing, running separate candidates in safe seats to create retaliatory disproportionality.

Exploitation of the overhang loophole has always been possible, but no party has tried it because the expectation is that voters will not put up with it. If Te Pāti Māori decides it’s worth the risk to try to deliberately undermine the proportionality of the election result, and this affects who governs, there will be a response. It could be bad news for the Māori seats, or for MMP generally. Or they could fail in their attempt because voters punish their candidate for attempting to play games with democracy.