New Zealand's Prime Minister John Key said he was "ecstatic" on Saturday after a crushing election win, securing his third term as leader of the South Pacific nation.

"I'm ecstatic, it's a great night," the leader of the centre-right National Party said.

Key confirmed his popularity with New Zealanders, increasing his vote despite a tumultuous campaign marked by allegations of dirty tricks and mass spying.

The 53-year-old's party won 61 of 121 seats, making him the first New Zealand leader to be able to govern in his own right since the country introduced proportional voting in 1996.

"This is a victory for those who kept the faith," he told supporters who chanted, "Three more years."

"This is a victory for those who refused to be distracted and knew that a vote for National was a vote for a brighter future."

Opposition leader David Cunliffe had quickly conceded defeat.

"We will not be able to form a government," Cunliffe told Labour supporters as National held around 48 percent of the vote.

"I have called John Key and congratulated him and acknowledged he will continue to be prime minister of New Zealand."

It was the third successive election defeat for the Labour Party, whose voter support, at 25 percent, was down nearly a third from where it was 12 months ago when Cunliffe replaced David Shearer as leader.

Cunliffe said he would not be stepping down despite the poor result, but said the party needed to "reflect very, very carefully" on what led to the defeat.

It was a campaign "beset by dirty politics and side-shows involving potential abuses of power at the highest level that will still take months and months to unravel," he said.