Prime Minister Jacinda Ardern said there were no excuses for Labour's botched handling of complaints, including a sexual assault claim, against a former party staffer.

"We have a duty of care and we failed in it," Ardern told media at her post-Cabinet press conference this afternoon.

She said it could happen to any party, and it was her job to address those mistakes and help other institutes to do the same.

She said the held two conference calls with the Labour Party's NZ council over the weekend.

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She said the complaints made to the party were serious and needed to be heard in a timely way. "That didn't happen."

The terms of reference for the review by Maria Dew QC had been finalised but would not be released at the request of the complainants.

Dew said she wanted to look at the substance of the complaints and not the Labour Party processes, Ardern said.

So the party's lawyer, who had looked into the party's process, would hand that work to a third party. That would go to all involved parties and asked for feedback.

Ardern said that prevention and training and victim-centred processes would be set up to improve party processes.

And she said she would meet with the complainants at their convenience, and those meetings would be guided by experts.

She said Poto Williams would lead work in conjunction with experts to "support culture change for everyone".

"Her work will be be informed by everything we learn in the next four to eight weeks."

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Ardern said these measures would not change how difficult the process had been for the complainants, but the next steps should improve the party's culture for the future.

She added that that change should have happened after the Berryman report that followed the sexual assault allegations at the Young Labour summer camp last year.

Ardern said a third party review would look at the paper evidence provided to the Labour Party, including whether complainants' evidence included a sexual assault complaint.

She said it was up to Maria Dew QC to take public the findings of her review.

The third-party review into Labour's processes would be made public on the condition that participants wanted that.

Ardern said the work would put in place best practice for the party and a victim-centred approach.

"For me it's about creating a process that people can have confidence in."

She said all parties would be able to respond to the summary of facts if they wished.

She did not put a timeline of the reviews' findings, saying it was more important to get the process right.

She would not say whether she expected further resignations from the party's New Zealand Council in the wake of Nigel Haworth's resignation last week.

She would not comment on Simon Mitchell's statement, released today, that he was not told about any sexual assault complaint when he was part of the party's investigating panel, though she added that public statements did not serve the complainants.

"I'm creating a space now where we can take this forward."

Ardern said the third party reviewer had not yet been appointed, but it would have to be someone all parties were happy with.

Ardern would not say whether any of her staff or any ministers had told her about a sexual assault claim and said she was focused on moving forward.

The ex-staffer at the centre of the allegations, who was employed by Parliamentary Service and worked in the Labour leader's office, quit his job last week but continues to say he has done nothing wrong.

His resignation followed that of Haworth, who stood down from the Labour Party's presidency after Ardern challenged him over his handling of the complainants and the harm that the party's process had caused them.

The Labour Party cleared the ex-staffer of wrongdoing in July, but Dew is now reviewing the issue after complainants said the process was unfair and the sexual assault complaint was ignored.

National deputy leader Paula Bennett is calling for Ardern's and Grant Robertson's actions to be included in the terms of reference because a key issue was what they were told about the sexual assault complaint and how they responded.

"It's the only way it's going to clear the whole thing up, and it's about justice for the complainants."

But she had no faith that their actions will be scrutinised.

Robertson was reportedly told about the sexual assault claim at the end of June, but he has not commented on when he was told about such a claim, citing the QC process and the complainants' privacy.

He said last week that he had not been contacted by Dew, but would speak to her if asked to.

Ardern has said that a report in The Spinoff on Monday was the first time she had seen such a complaint from one of the complainants, suggesting that any previous complaint she was told about had been laid second- or third-hand.

She has said that the terms were being drawn up with the complainants, and their wishes should be paramount.

Ardern has remained tight-lipped about the terms of the ex-staffer's resignation, saying on Friday that she would not comment beyond saying she had received the resignation.

The ex-staffer's contract with employer Parliamentary Service would have included a "relationship breakdown" clause which, if invoked by Ardern, would usually have entitled him to three months' salary.

Bennett said that Ardern should come clean and say whether the ex-staffer was getting a payout.

Last week was a tumultuous one for the Labour Party, with Haworth and the complainants offering contradictory claims over whether a sexual assault complaint had been directly laid with the party.

Haworth said that neither he nor the party's panel investigating the complaints were aware of one, but several complainants disagreed and said that there was an email trail to support their claims.

Ardern has said that the Dew review, expected to report directly to Ardern in three weeks, would get to the bottom of it.