There are calls for the Labour Party's president to resign over his handling of allegations of sexual assault against a staff member amid an investigation into the saga.

Meanwhile, lawyers for the man at the centre of the accusations say they are "without foundation".

The party has been rocked by allegations published in The Spinoff this week that a 19-year-volunteer was subjected to a prolonged sexual attack by a male Labour staffer in February 2018.

The staffer was investigated by the party over complaints from seven different people earlier this year, but it concluded no disciplinary action was needed. Following complaints last month, the party has now asked a QC to review the investigation.

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Labour says it did not investigate claims of sexual assault and that it was not the appropriate body to do so. The volunteer is reported to have said she had met with Labour president Nigel Haworth in 2018 and described the alleged attack to him, and in March 2019 told the investigating panel about it.

An open letter to Prime Minister Jacinda Ardern and Labour's major governing bodies is now calling for Haworth to resign over the matter.

Jacinda Ardern on alleged sexual assault by a Labour staff member. Video / Mark Mitchell

Posted online, the document says it has been written by the Labour Party, some complainants and other supporters.

"We call on Nigel Haworth to formally resign as President of the Labour Party and to withdraw his candidacy for re-election at the November Conference," it reads in a list of eight demands.

"All of us have watched in horror as this story has unfolded, as the survivors have been repeatedly re-traumatised, and as the Labour Party has run a shambles of a process."

It also calls for a formal apology from Haworth, changes to the party's sexual harm prevention policy, training for all party members on sexual harassment prevention and for the staffer to be barred from the Parliamentary precinct.

In a statement to the Herald, a lawyer for the staff member facing allegations said he had confirmed the claims were without foundation.

"My client was the subject of inquiry, and that inquiry concluded that there was no case to answer," he said.

"He now finds himself in the distressing situation of being subjected to a further inquiry. He has agreed to co-operate fully with that further inquiry, and given that this process is currently being conducted, I have advised my client that it is completely inappropriate for these matters to be litigated in the media."

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Ardern on Monday told media she had only learned the allegations against the staffer were of a sexual nature of that day and that the party had previously assured her they were not.

She said she was deeply frustrated and concerned about the party's handling of the situation and the conflicting report, would not say she had confidence in Haworth, and said she would wait for the outcome of the QC's report.

Ardern in a series of interviews on Tuesday morning again repeatedly refused to back the president.

Asked by Newstalk ZB if she would sack Haworth, Ardern said if a report found the party hadn't dealt with the matter appropriately "or that there's been failures on his part, he would do what's in the best interests in the party and those involved".

Haworth has declined repeated requests for comment.

Ardern on Monday said the staffer had not been working at the Parliamentary precinct for about five weeks and would not return, at least, until the review was complete.

In a statement, Parliament Speaker Trevor Mallard said no complaint had been made to Parliamentary Service - the man's employer - by anyone involved in the case, either.

"I repeat my request for any individual who feels unsafe at work or when visiting to contact me or the general manager. Any further action requires a complaint," he said.

WHAT HAPPENED WHEN?

February, 2018: A Labour staffer - employed by Parliamentary Service - allegedly sexually assaults a 19-year-old Labour volunteer after a meeting at his house, according to The Spinoff.

March, 2018: Complaints emerge about Labour's handling of assault allegations at a Young Labour summer camp earlier in the year and the party launches a review.

April, 2018: Spurred by the review, the volunteer emails the lawyer leading a probe into the camp incident, Maria Berryman, alleging a "lower-level" 2017 incident involving the staffer but is told the review is looking in the camp issues first.

August, 2018: The volunteer meets with Labour Party president Nigel Haworth and tells him about the sexual assault, Stuff reports.

February, 2019: Labour's NZ Council orders an investigation into the complaints about the staffer from the volunteer and six other people.

March, 2019: The volunteer and the other complainants testify and, according to the Spinoff, the volunteer describes the alleged sexual assault.

July, 2019: The review emails the complainants telling them no disciplinary action will be taken.

July 12, 2019: Email from a third party sent to journalists raising concern for the complainants.

August 5: Reports emerge about allegations of "bullying, sexual harassment and sexual assault", and resignations from the party. Media are told the matters looked into do not involve sexual assaults.

August 15: The party announces it will let the complainants appeal.

September 9: A graphic description of an alleged sexual assault by the volunteer is published by The Spinoff.

September 10: In a statement, a lawyer for the staff says the allegations are without foundation.