How an overhang could change the election result

Summarised by Centrist

An overhang occurs when a party wins more electorate seats than its party vote entitles it to. Those elected MPs keep their seats, and Parliament expands beyond its usual 120 members.

Under MMP, the party vote normally determines a party’s total seats. Electorate MPs count towards that allocation, with list MPs filling the balance. Winning electorates ordinarily changes who enters Parliament, rather than adding seats to a party’s entitlement. 

For example, if a party qualifies for four seats through its party vote but wins six electorates, it keeps all six. The additional two seats create an overhang. Assuming no other adjustments, Parliament grows to 122 MPs, requiring 62 for an outright majority. 

This creates an opportunity for tactical voting. Supporters can elect one party’s electorate candidates while giving their party votes to an ally. If the first party retains enough electorates, its seat count can remain unchanged while its ally gains seats.

Te Pāti Māori encouraged this approach at its campaign launch. Author Oliver Hartwich estimates it could deliver three or four extra seats for the left bloc under the polling assumptions used.

A report by the NZ Initiative also models National winning more electorates than its party vote allocation covers. In that situation, some supporters could transfer their party votes to coalition partners without reducing National’s electorate representation.

Under the report’s assumptions, shifting one in nine National party votes delivers a government majority. Shifting one in four produces 69 coalition seats in a 132-member Parliament, despite opposition parties collectively receiving more votes.

Economist Oliver Hartwich proposes the Electoral Commission balance overhang seats with additional seats for other parties to restore proportionality. That would reduce the potential advantage from coordinated vote splitting while preserving electorate winners’ places in Parliament. 

Read more at The NZ Initiative

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