Jacinda Ardern has announced she will quit as New Zealand prime minister next month, saying she no longer has "enough in the tank" to lead.

Ms Ardern choked up as she detailed how six "challenging" years in the job had taken a toll.

She will step down as Labour Party leader no later than 7 February. There will be a vote in the coming days to determine her replacement.

New Zealand will hold a general election on 14 October.

Ms Ardern, 42, said she had taken time to consider her future over the summer break.

"I had hoped that I would find what I needed to carry on over that period but, unfortunately, I haven't, and I would be doing a disservice to New Zealand to continue," she told reporters.

Ms Ardern became the youngest female head of government in the world when she was elected prime minister in 2017, aged 37.

And a year later she became the second world leader ever to give birth while in office.

She has led New Zealand through the Covid-19 pandemic, the Christchurch mosque shootings, and the White Island volcanic eruption.

"It's one thing to lead your country through peace time, it's another to lead them through crisis," she said.

"These events... have been taxing because of the weight, the sheer weight and continual nature of them. There's never really been a moment where it's ever felt like we were just governing."

Her domestic popularity has declined to new lows in recent months, according to opinion polls. But Ms Ardern said she was not resigning because she believed Labour could not win the election, but because she thought it would.

"We need a fresh set of shoulders for that challenge," she said.

Australian Prime Minister Anthony Albanese paid tribute to Ms Ardern as a leader of intellect, strength and empathy.

"Jacinda has been a fierce advocate for New Zealand, an inspiration to so many and a great friend to me," he wrote on Twitter.