As the pitch of the election campaign intensifies in New Zealand, a very particular species of political phenomenon has emerged: so-called strategic (or tactical) voting and its potential consequence, a parliamentary overhang.

This isn’t new, but the issue is creating more clamour than usual at this election because of the nature and scale of the strategic voting being advocated.

New Zealand’s mixed member proportional (MMP) system allows voters to choose their preferred political party via their party vote, but also use their second vote to select an electorate candidate – who may be from a different party.

And because of the “coat-tailing” rule, even parties that fail to make the 5% threshold for parliamentary representation – but win one or more electorate seats – are allocated a share of seats proportionate to their party vote.

Most voters give both votes to the same party. But vote splitting has been going on since the first MMP election in 1996, when New Zealand First candidate Tutekawa Wyllie took the Māori electorate of Te Tai Tonga while Labour won the party vote.

The best-known example was in 2011 when then-Prime Minister John Key had his famous cup of tea with then ACT leader (and former National MP) John Banks to signal he “wouldn’t be unhappy” if National voters gave Banks the Epsom electorate but their party vote to National.

That arrangement stayed in place for several elections and was pivotal to the parliamentary survival of the ACT party.

Vote splitting does not always produce overhangs. But when it does, to maintain overall........

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