New Zealand Prime Minister Jacinda Ardern on Thursday made a shock announcement she had "no more in the tank" to continue leading the country and would step down no later than early February and not seek re-election.

Ardern, holding back tears, said that it had been a tough five and a half years as prime minister and that she was only human and needed to step aside.

"This summer, I had hoped to find a way to prepare for not just another year, but another term - because that is what this year requires. I have not been able to do that," Ardern, 42, told a news conference.

"I know there will be much discussion in the aftermath of this decision as to what the so called 'real' reason was... The only interesting angle you will find is that after going on six years of some big challenges, that I am human," she continued. "Politicians are human. We give all that we can, for as long as we can, and then it's time. And for me, it's time."

A ruling New Zealand Labour Party vote for a new leader will take place on Sunday; the party leader will be prime minister until the next general election. Ardern's term as leader will conclude no later than 7 Feb and a general election will be held on 14 Oct.

Ardern said she believed Labour would win the upcoming election.

New Zealand Deputy Prime Minister Grant Robertson, who also serves as finance minister, said in a statement he would not seek to stand as the next Labour leader.

Political commentator Ben Thomas said Ardern's announcement was a huge surprise as polls still ranked her as the country's preferred prime minister even though support for her party had fallen from the stratospheric heights seen during the 2020 election.

Thomas said that there was not a clear successor.

Ardern said she was not stepping down because the job was hard, but because she believed others could do a better job.

She made a point of telling her daughter Neve that she was looking forward to being there when she started school this year and told her longtime partner Clarke Gayford that it was time they married.

Her initial election made a big splash on the global stage because of her gender and youth, coining the phrase "Jacinda-mania".