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Mr Key has said the country would benefit from a four-year parliamentary term but the change needed to be supported by the public via a referendum.
The proposal failed when it was put to voters in the 1967 and 1990 elections, with almost 70 per cent favouring the three-year term over four years.
Mr Key told TVNZ's Breakfast this morning that there could be a different outcome if it went to a referendum now.
"Twice in New Zealand's history we've voted on a four-year term. Both times it was rejected roughly two thirds [to] one third but I think the mood's definitely changing with MMP."
He said 2017 was the earliest we would see a four-year term introduced.
"My view of it is we hardly ever have one-term governments in New Zealand. Three years, whether you love or hate the colour of the government that's in office at the time, is a bit too short to properly assess them, and actually elections are expensive and slow the process down, so four years would be better than three."
The Labour Party and United Future both support the proposal of a four-year term, while the Greens and New Zealand First have acknowledged the merits of the idea.