Wellington: The New Zealand government’s handling of a fragile economy was the “overwhelming” reason for its third election victory, Prime Minister John Key said Sunday after his party won a historic majority and he eyed a fourth triumph.

Voters “saw the economic direction we wanted to continue to take the country in and the things we have achieved over the last six years”, the centre-right leader said.

Asked if the economy was behind his resounding win in Saturday’s election, Key replied: “Yes. Overwhelming.”

And he vowed more of the same.

“I don’t intend to take the party veering off to the right,” Key said. “We’ve held the centre ground for the last six years. We’re not looking to do radical things.”

Accusations of dirty politics and mass spying dominated the hustings but had little impact on the result as Key’s National Party increased its support.

It was the first time in the seven elections since New Zealand adopted a complicated proportional representation system that a party has won enough seats to govern alone.

But Key said he would not forget his coalition partners of the past two elections — ACT, United Future and the Maori Party — and would offer them roles in the new administration.

“I always believed one of the reasons we did hold our support was because in the worst of economic times we really did continue to fund, and had to borrow to fund, programmes to support the most vulnerable New Zealanders and I know there’s more work to be done,” he said.

The central bank estimates the economy will grow 3.7 per cent this year and budget figures say this should increase to 4.0 per cent next year, while inflation in the year to June was a benign 1.6 per cent.

The government also posted its first budget surplus in six years in May, and says it has a plan to keep spiralling debt under control.

As the opposition left-wing parties licked their wounds following the rout, Key promised an administration “that governs for all New Zealanders” and even contemplated contesting a rare fourth term in office at the next election in 2017.

“Well, I’d like to,” he said. “You’ve got to believe we’ve got a great chance of doing that but I’m not silly enough (not) to know that to get that fourth term we have to deliver over the next three years.

“Fourth term sounds great, but you’ve got to earn it.”

A fourth consecutive victory has only been achieved twice in New Zealand — by the National Party led by Keith Holyoake in the 1960s and the Liberals under Richard Seddon who won five elections from 1893.

Key’s Nationals won 61 of 121 parliamentary seats, up from 59 at the 2011 election, while the main opposition Labour Party managed only 32, down two, in its worst performance since the 1920s.

To maintain stability, Key said he would be speaking to the coalition partners in his previous government who could provide a further four seats under the mixed-member proportional (MMP) system.

“Technically on the numbers we don’t need other partners but MMP is a system designed to work with other political parties and I think that would be a nice grouping to carry forward,” he said.

It was a bleak poll for Labour, but defiant leader David Cunliffe refused to step down.

“Of course I take responsibility (for the loss) ... but at the end of the day we need to move forward, and our campaign to win in 2017 starts right now,” he said as Labour faces at least nine years sidelined, starting from Helen Clark’s third-term loss to Key in 2008.

But the knives could still come out. Former Labour leader David Shearer and his ex-deputy Grant Robertson refused to rule out standing in a leadership challenge. Current deputy leader David Parker would not answer directly when asked if he would endorse Cunliffe.

Voter turnout was 77.04 per cent, slightly higher than the 74.2 per cent in the previous election, but the third lowest in the past 100 years.