Ardern standing down as New Zealand PM

Prime Minister Jacinda Ardern has shocked colleagues and New Zealanders by announcing her retirement from politics.

Ms Ardern choked back tears on Thursday as she said she spent the summer considering her future, concluding she did not have the energy to seek re-election.

"I'm entering now my sixth year in office and for each of these years I have given my absolute all," she said.

"I know what this job takes. And I know that I no longer have enough in the tank to do it justice.

"This summer I had hoped to find a way to prepare not just for this year but for another term ... I have not been able to do that.

"I would be doing a disservice to continue."

Speaking from Napier, where Labour's caucus had gathered for a year-starting retreat, Ms Ardern said she left without regrets and offered a simple way she would like to be remembered.

"As someone who always tried to be kind," she said.

Ms Ardern had been expected to announce an election date - which she did, for October 14 - but then stunned all-comers by declaring her exit from politics.

Ms Ardern also addressed her family - her fiance Clarke Gayford, who sat in on her press conference, and her four-year-old daughter Neve.

"To Neve, mum is looking forward to being there when you start school this year," she said.

"And to Clarke, let's finally get married."

New Zealand could have a new prime minister as soon as Sunday when Labour's caucus will meet in Wellington to consider the party leadership.

Ms Ardern informed her cabinet of her decision in an early-morning meeting and told Labour MPs before her press conference.

There are no confirmed contenders for the party leadership, with deputy prime minister Grant Robertson ruling himself out while paying tribute to Ardern.

"It has been the honour of my working life to have supported Jacinda as minister of finance and as deputy prime minister," he said.

"Her intellect, judgement and empathy mark her out as one of New Zealand's finest leaders.

"As a colleague, a friend and a New Zealander I am incredibly grateful for her service and commitment and wish her every joy and success in the future."

The race to succeed Ms Ardern as prime minister appears to be an open contest with education minister Chris Hipkins, a close ally, among the most likely.

Other contenders include previous leader Andrew Little, foreign minister Nanaia Mahuta and immigration minister Michael Wood.

Colleagues were left reeling by the bombshell announcement.

"I wasn't expecting it. It was a hell of a surprise," Emergency Management Minister Kieran McAnulty said.

"She's slugged her guts out for five and a half years for this country and I hope people take a step back and acknowledge that because she deserves it."

First elected in 2017, Ms Ardern rode a wave of "Jacindamania" to become prime minister weeks after taking the Labour leadership.

The 42-year-old became known worldwide for her compassionate brand of leadership, comforting New Zealanders in the wake of the country's worst terror attack in 2019.

She led Labour to a thumping re-election in 2020 in a campaign defined by her government's COVID-19 management.

On Thursday, she announced an election date for the 2023 campaign - October 14 - insisting Labour could still win.

"I am not leaving because I believe we can't win the election but because I believe we can and will need a fresh set of shoulders for that challenge," she said.

Under Labour party rules, two-thirds of the caucus must support a candidate for the leadership at Sunday's vote or the contest will be thrown open to the wider membership.

Ms Ardern - who will continue as MP for the Auckland electorate of Mt Albert through to April - has requested this process be complete by February 7, the day after Waitangi Day.