When is the NZ prime minister election and how will it work?

New Zealand Prime Minister Jacinda Ardern will step down no later than Feb 7 after a shock resignation on Thursday in which she said she "had no more in the tank".

Leadership hopefuls will gather on Sunday when the centre-left Labour party caucus meets to elect a new leader, who will become prime minister. Here's what will happen:

Who can vote, and how?

The 64 Labour members of parliament, known as the caucus, will meet in private at 1pm local time (0000 GMT) on Sunday to elect a new leader. Those who can't make it can vote by proxy. The winner requires at least two-thirds of the votes, or 43 members.

Hopefuls need seven colleagues to make the ballot: at least 10% of the caucus, excluding themselves. Nominations must be received by 9am local time on Saturday.

With Labour holding a majority in parliament, whoever they elect will become prime minister, helming the country for a little more than eight months before a general election set for Oct 14.

If the general election were held today, Labour would lose power, according to a Taxpayers Union-Curia poll released on Friday using data from before Ardern stepped down.

What if the caucus can't decide?

They keep going. Multiple votes are allowed, with the lowest polling candidate eliminated each round. The caucus has until Jan 26 to decide.

If the caucus is unable to pick a new leader by then, the vote will be expanded to include party members and affiliated unions. The caucus vote will be weighted at 40%, party members at 40% and unions at 20%.

Who arre the frontrunners?

There are four. Immigration, Police and Transport Minister Chris Hipkins is a household name for his role leading the government's Covid-19 response. He declined to rule out running on Thursday.

Foreign Minister Nanaia Mahuta is also expected to run after 26 years in parliament.

Transport Minister Michael Wood and Minister of Justice Kiri Allan are newer faces in parliament, elected in 2016 and 2017, respectively. Allan would become the first openly gay leader if elected.