Prime Minister Jacinda Ardern will not give details on how her party ended up voting for a clause to entrench part of the Three Waters legislation, a move heavily criticised by constitutional experts and which Ardern now admits was a mistake.

Ardern and her Local Government Minister Nanaia Mahuta have both said that the matter went to a Labour Party caucus meeting, although it appears the full detail of the proposal was not discussed there.

It is equally unclear whether the lack of information presented to caucus was a deliberate tactic to curry support for something MPs would usually oppose, or a simple mistake.

Speculation has now moved to whether Mahuta will keep her job in a forthcoming reshuffle of ministers - speculation Ardern inadvertently flamed during her post-Cabinet press conference.

Ardern said there had been discussions over a Labour Party proposal to entrench anti-privatisation parts of the Three Waters legislation at 75 per cent, meaning they could only be repealed after a successful referendum or if 75 per cent of MPs voted for repeal.

That move was scuppered by National, who refused to use their votes to support entrenchment. National’s votes were necessary to entrench the legislation at a threshold of 75 per cent.

What is less clear is whether Labour MPs discussed a new proposal from the Greens to entrench at 60 per cent, which they could do with just Labour and Green votes.

This was the proposal Labour MPs eventually voted to support.

Ardern would not say whether the 60 per cent entrenchment proposal was raised with her before it went to Parliament.

“As I have said, I am not going to get into the individual discussions in caucus,” Ardern said.

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“The principle of entrenchment was discussed, and as I have said it is commonly understood to be a much higher threshold than what came through in the debate and was voted on - it was a mistake and it will be fixed,” she said.

Asked whether Mahuta gave sufficient information to her caucus colleagues at the meeting in question, Ardern said the amendment had not been tabled at the point at which caucus met.

The SOP was tabled by the Greens the day it was voted on, so the specific wording of the SOP might not have gone to Labour’s caucus, which meets on a Tuesday.

However, the Greens have previously said that they were clear on what the SOP would contain. The Greens point to the fact their proposal was included in a report from the Finance and Expenditure Committee, which was published on November 11, weeks before the vote.

This report was written with Labour MPs who hold a majority on the committee.

The Green Party’s contribution to that report pointed out both the fact that officials warned of the constitutional impacts of the move and the report explicitly mentions the Greens’ disagreement with this and preference for entrenching at 60 per cent.

“Early proposals to entrench public ownership through a 75 per cent majority are now described by the Department of Internal Affairs as inappropriate for legislation unrelated to constitutional matters.

“The Green Party disagrees that entrenchment should only be used for constitutional matters, and considers entrenchment with a 60 per cent majority (given the lack of support from National for a 75 per cent per majority) would be appropriate as a check on future privatisation of water infrastructure,” the report said.

Labour MPs, particularly those on the committee and Mahtua as the minister responsible, should have read that report before the caucus discussion.

Mahuta’s office even put out a press release on November 11 announcing the publication of the report.

Ardern would not say whether Mahuta should have raised the issue with caucus if she had known the proposal was to entrench at 60 per cent.

“I can see that the issue here is each among you is looking for one individual person to blame here, the point I am making is that as a team we are taking responsibility for the fact that we voted for another party’s amendment,” Ardern said.

“As you well know not every SOP will even have an opportunity to go to caucus. I’ve acknowledged there was a general discussion, I’ve not gotten into the detail of that to preserve caucus,” Ardern said.

“Regardless, we’ve acknowledged that there was a mistake and we’re fixing it,” she said.

At her post-Cabinet press conference, Ardern hinted Mahuta might not keep her portfolio until the election. Ardern is planning a reshuffle of portfolios early next year.

When asked whether the local government minister would keep her job up to the election, Ardern replied, “yes, because we will win the election” - apparently mishearing the question as whether the minister would keep her job after the election.

However, when asked to clarify whether Mahuta would keep her job until the election, Ardern equivocated.

“I have not set out any changes at this, but I would not have you speculate on that based on the fact that we haven’t formulated our reshuffle. I have no plans on changing the minister based on anything that has happened here,” she said.