New Zealand needs a new prime minister, and the Labour Party a new leader.

Prime Minister Jacinda Ardern shocked the country and her Labour Party colleagues on Thursday, unexpectedly announcing she would resign in the coming weeks.

But the country will not decide on its next leader until an election, set down for Saturday, October 14. Instead, Labour MPs will gather this weekend to choose the party's, and New Zealand's, new leader.

Deputy Prime Minister Grant Robertson has already ruled himself out. The party's deputy leader Kelvin Davis has also ruled himself out, as has newcomer to Cabinet Kieran McAnulty, seen by many to be a rising star.

No-one else has yet declared they will contend, but some have declined to rule themselves out.

So who could replace Ardern?

Chris Hipkins

Education and police minister Chris Hipkins is among senior Labour MPs with the most experience in Cabinet. He has a reputation for being a fixer of problems, and has something of a national profile after fronting the Covid-19 response for some of the pandemic.

He was parachuted into the police portfolio in 2022 as Ardern sought to reset the Government's law and order efforts amid a spate of high-profile crimes and attacks from the Opposition.

Hipkins, the MP for Remutaka in Wellington's north, has been an MP since 2008, and is currently ranked fifth in Cabinet.

He would not rule out running for the leadership when asked on Thursday.

'No, I'm not [ruling out running] but I'm also saying that, you know, at the moment, I think people just need a little bit of time,” he says.

'In due course, I think I need to have a conversation with my colleagues. As do my colleagues need time to digest the news.”

Michael Wood

A comparative newcomer to politics, Michael Wood has been a strong performer in Cabinet with a reputation as a safe pair of hands.

As workplace relations minister he has ushered through a hallmark Labour policy, fair pay agreements, on top of managing the transport and immigration portfolios.

He entered Parliament in 2016 as MP for Mt Roskill.

Wood did not rule out running for the leadership on Thursday.

'I'm very proud to have served under Jacinda Ardern, one of the finest prime ministers we'd ever had, for the past couple of years. Today's about recognising her extraordinary achievements. That's all I'll be saying,” he says.

Megan Woods

Housing Minister Megan Woods is a senior Labour MP with a long history in politics who will again run the party's election campaign in 2023.

Housing has been a major policy area for Labour and, after the Government's failed attempts to build homes after coming to power in 2017, Woods was tasked with getting this work on course.

She was also appointed minister in charge of managed isolation facilities during the pandemic, as failures in the system emerged.

Woods was elected to the Christchurch electorate of Wigram in 2011, joining the Labour Party in 2007 after years in the Alliance and Progressive parties.

Nanaia Mahuta

Foreign Affairs and Local Government Minister Nanaia Mahuta contested the Labour leadership in 2014, and is among senior Labour ministers who represent the party's old guard.

Mahuta has significant clout with Labour's Māori caucus and has a high profile.

However, she has fronted some of the parties most contentious policies, namely Three Waters reform and co-governance between government and iwi representatives within this reform. This has made her a lightning rod for controversy, and may be seen as a risk by other MPs.

Kiri Allan

A fast riser in Labour's Cabinet, Justice Minister Kiri Allan may be an outside contender for the leadership.

Allan entered Parliament as a list MP in 2017, and won the East Coast electorate in 2020.

A former lawyer who was also involved in the agriculture sector on the East Coast, Allan has been among the top performers of Labour's newer MPs and has held numerous ministerial positions since 2020.

She was conservation and emergency management minister before taking on the justice portfolio in June 2022. She also has an associate finance portfolio.

Allan could represent a fresh start for the party as it heads to the 2023 election.

- Thomas Manch, Nikki Macdonald and Anna Whyte/Stuff.