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Haworth has maintained that at no stage through the party's drawn-out investigation was it clear that the complaints involved a sexual assault.

Ardern said she had been assured at the time by her party bureaucracy the allegations were not serious.

But the scandal took a turn after fresh details, reported by The Spinoff website, came to light.

"In the last 48 hours I have read incredibly distressing reports of an alleged sexual assault involving members of the Labour Party," Ardern said on Wednesday.

"The party was never equipped to appropriately deal with the issue."

The positions of three yet-to-be-named Labour officials, who conducted the investigation, also appear untenable.

Labour's investigation began in October last year and concluded in July, when Haworth emailed complainants advising of the decision to exonerate the staffer, offering no lines of appeal.

That enraged the Labour members enough to take their misgivings to opposition deputy leader Paula Bennett.

Bennett promoted an open letter from "MeToo Labour" on behalf of the complainants, which requested Haworth's resignation and for Ardern to enact other reforms.

Bennett upped the stakes in Parliament on Wednesday, naming members of Ardern's senior staff who she said were aware of the allegations and chose to "draw a curtain over sexual misconduct and behaviour".

Ardern said that upon learning of the sexual nature of the allegations last month, she intervened to install a Queen's Counsel to investigate the claims.

"I have made it clear that I want the QC-led appeal process to resolve this matter," she said.

Ardern apologised to the alleged victims, and extended an offer to meet. The scandal is a particularly thorny one for the prime minister, who has placed herself within the 'Me Too' movement that aims to call out and end sexual harassment and assault.

"Raising an allegation of sexual assault is an incredibly difficult thing to do; for additional distress to be caused through the way those allegations are handled is incredibly upsetting," she said.

"On behalf of the Labour Party I apologise to the complainants for the way this matter has been dealt with.

"I would be happy to meet with complainants ... I will take steps to make that offer available to them if they wish to take it up."

"I also want to assure myself that appropriate victim support and advocacy have been put in place around the complainants."

The complainants have not detailed their allegations to the police.

AAP