Labour's Chris Hipkins faces leadership vote - what does he need to survive?

Sixty percent of the votes cast - plus one.

That is what Chris Hipkins needs to survive as the Labour leader.

Newshub understands his leadership will be voted on at a caucus meeting in Upper Hutt on Tuesday.

It's the first time the new Labour caucus will meet since the release of the final election results and will see Hipkins face what essentially is a vote of confidence.

Under Labour's constitution, there must be a vote on endorsing the leader within three months of a general election.

Following a caucus meeting right after the election, Hipkins said Labour MPs agreed to hold the vote once the final results were known, as to allow the new caucus the power to endorse - or not to endorse - the leader.

That time has now come.

"The leader is the leader until they're not and I'm certainly still the leader of the Labour Party," Hipkins said in October.

"I've still got a bit of fight left in me."

According to the party's constitution, the number of votes required to endorse the leader is 60 percent of those cast, plus one. With a caucus of 34, it means Hipkins needs 21 votes to remain as leader if all MPs vote.

If he gets fewer than that, the leadership is vacated. MPs then have seven days to endorse a new leader (66.66 percent of the votes are needed) or an election is triggered.

In the event MPs can't get two-thirds support for a new leader, Labour goes to its complicated electoral college, which is comprised of caucus members, party members and affiliates (unions). Caucus votes having a weighting of 40 percent, party members are 40 percent, and then affiliates 20 percent.

This essentially pits contenders against each other in the public eye. A winner needs to get a majority of support, which if there are more than two candidates, can go on for several rounds.

The last such contest, in 2014, ended in a very tight result. While Grant Robertson was the favourite of both the caucus members and the party members, Andrew Little's support among union affiliates got him over the line. Little won with 50.52 percent of the overall vote to Robertson's 49.48 percent.

Neither Dame Jacinda Ardern nor Hipkins had to go through such an ordeal. Both were the only nominees in their respective leadership elections. 

Labour is now entering Opposition after six years in Government. It received just 26.91 percent of the vote, which gives it 34 seats.

Hipkins became the Labour Party leader in January after the resignation of Dame Jacinda. While he remained the preferred-Prime Minister in the Newshub-Reid Research poll for most of the year, he was overtaken by National's Christopher Luxon just prior to the election.