Labour's president has quit after a pointed phone call from the Prime Minister, and Newshub can reveal more details about how much Grant Robertson and Nigel Haworth knew.

Haworth refused to comment when Newshub approached him in Auckland's Westmere on Wednesday. He left it to the Prime Minister to apologise for him.

"I do want to offer an apology on behalf of the Labour Party to those complainants in this case who have gone through a process that I believe has caused them harm," Jacinda Ardern said.

Haworth insists he didn't know about allegations of sexual assault against a Labour staffer. But the complainants say that's rubbish.

A complainant told Newshub: "It takes immense strength and bravery for victims of sexual assault to come forward in the first place, but to blatantly hear him lying on the news about knowing what we told him is absolutely despicable."

Newshub has been told the young woman told Haworth first-hand about the alleged assault.

"She watched him squirm in his seat and looked so uncomfortable while she detailed the sexual assault to [Haworth]," the complainant said.

And Newshub has obtained several email chains to Haworth from victims which mention sexual assault.

One email is dated 12 October last year, five months before the allegations were investigated, with the subject line "Follow up on SA allegations".

Another on 18 October mentions sexual assault.

And on 21 May, two months after the investigation, a complainant says, "just reiterating that the accusation (includes) assault".

They go on to say, "I feel threatened, I feel insecure and I feel unsafe... I can't deal with this anymore."

The Prime Minister said she has seen correspondence that has "clearly" shown her that "despite best intentions, this process has caused harm and that is not acceptable".

National leader Simon Bridges said it was unacceptable that it took Ardern so long to respond.

"Five weeks ago she was asked about this. That should have set a responsible person - let alone the Prime Minister - on a course of action to work this out."

Complainants also say Robertson, the Finance Minister, was told in person about the sexual assault allegations on 30 June at an event where the Prime Minister was also in attendance.

Newshub revealed in August Robertson was aware of the investigation and some complaints.

"I'm going to respect the privacy of the young people involved in this situation," Robertson said on Wednesday, when asked if he was told.

"I'm simply not going to go into it."

The Prime Minister was asked if Robertson ever raised sexual assault allegations with her. She responded saying the party "has maintained the same position".

Robertson said he's "comfortable" with what he's done throughout the process.

Newshub.