At least four members of the Labour Party have reportedly resigned over an investigation into allegations against a staff member.

However, in an interview with Newstalk ZB's Mike Hosking this morning, Prime Minister Jacinda Ardern refused to confirm the exact number of people who had left.

MediaWorks yesterday reported seven party members and officials laid complaints this year accusing the senior party staffer of "unacceptable behaviour ranging from bullying and intimidation to sexual harassment and sexual assault".

When pressed by Hosking, Ardern said she wouldn't be commenting on the investigation, labelling it a "party matter". She also refused to comment on the number of people who had left.

"This is an inquiry which is still under way and is a party matter ... but if you want numbers who come and go from the Labour Party I can't tell you.

"I've seen the claims but I can't give you verified numbers.

"It's something for the party and not something that I am directly involved in ... the party has already indicated that it is checking that everything it should have done was done when the complaints [were made]."

As for what was now happening, she said the party was "currently assessing the inquiry which was undertaken".

Asked why she wouldn't inquire personally, Ardern replied that it wasn't appropriate.

As for when the inquiry would be completed she said that was also "a matter for the party".

However, Newstalk ZB political reporter Barry Soper said the allegations involved not only sexual assault but rape, offers to pay for sex and that there were 12 alleged complainants.

"What they're saying is, the party is telling them not to comment. They say people incredibly high up are on the side of the accused in the Beehive."

Soper said those allegedly affected were "frightened" of talking to the media as they claimed they would have their support withdrawn if they spoke out.

"They talked about not just sexual assault but rape, they said that this person worked in the Beehive for two years, he was offering promotions for sex. They said there was a huge power imbalance, they said the person was well known to Jacinda Ardern and Grant Robertson.

"They said one victim was hospitalised, two were still working at Parliament, six others were junior party members and there were 12 victims in all."

Soper said the complainants just want the party to be held to account.

He had tried to get comment from Labour Party president Nigel Haworth.

"He said a firm no comment and that was the end of the matter."

However, Soper said he'd discovered the man at the centre of the allegations had hired a lawyer who told him "that he is innocent, there was an inquiry, he has nothing to answer for and they warned us to leave this issue alone otherwise they will be suing".

Soper said the alleged offender worked for the Labour Party as opposed to Parliamentary Services.

He was unsure when the offending started but there had been mention of a Christmas Party "where bad behaviour occurred".

The whistleblower said they wanted to speak out as the Labour Party had already investigated and with the alleged offender having support of people "high up in the party", "they'd felt that this matter was not going to go any further".

Asked by Hosking why the complainants haven't gone to police, given the seriousness of the allegations, Soper said they probably could "but I think they felt that they were loyal party members and that the party could sort this".

"But these allegations are very serious indeed. When I originally got the first email about a month ago it couldn't be ignored but any attempt to follow it up with the party was fruitless."

Soper said it appeared the claims were separate from those made in relation to a summer camp which involved a 20-year-old accused who had two charges against him dropped following a police investigation into an incident at a Labour Party summer camp at Waihi in February last year.

Meanwhile, MediaWorks last night reported the staff members' complaints included "unacceptable behaviour ranging from bullying and intimidation to sexual harassment and sexual assault".

It said the complaints were investigated in March and no disciplinary action was taken. It said the man's lawyer said the allegations were untrue.

"But after concerns were raised about the investigation, the party is now reviewing its processes - but not the outcome," MediaWorks reported.

It said at least four people had resigned from official party roles and cancelled their membership as a result.

MediaWorks said the complainants said the Labour Party had done nothing to improve its processes since last year's allegations of sexual harassment at the Young Labour summer camp.

A 20-year-old man was charged with four counts of indecent assault at the Waihi camp.

An inquiry by Wellington lawyer Maria Austen recommended that the party should review its code of conduct and develop policies on sexual harassment, sexual assault, alcohol, events, host responsibility and bullying.

However Prime Minister Jacinda Ardern refused to release the report, arguing that there were matters before the courts.

MediaWorks said it had spoken to five people who said nothing had changed since the report was released.

The people who complained about the staff member's actions this year told MediaWorks that they suffered panic attacks and vomiting and had to take days off work because they were so scared of the staffer.

"As far back as August 2018, Labour Party president Nigel Haworth was informed the staffer told a woman she was elected within the party because she 'would be nice to sleep with'," MediaWorks said.

"He was also told a senior ministerial staffer had warned an alleged victim that if she told anyone about the staffer's behaviour she would be shut down."