Labour Party leader Jacinda Ardern has accepted Party President Nigel Haworth's resignation.
In a statement, Ms Ardern said: "In the last 48 hours I have read incredibly distressing reports of an alleged sexual assault involving members of the Labour Party.
"This morning I was provided some of the correspondence from complainants written to the party several months ago. It confirms that the allegations made were extremely serious, that the process caused complainants additional distress, and that ultimately, in my view, the party was never equipped to appropriately deal with the issue.
"I discussed the correspondence with the Labour Party President this morning. Whilst he stands by the statements he has made on this matter I believe mistakes were made."
Ms Ardern acknowledged that raising an allegation of sexual assault was "incredibly difficult to do" and said additional distress caused by the way those allegations were handled was upsetting.
"On behalf of the Labour Party I apologise to the complainants for the way this matter has been dealt with," she said.
She reiterated that she wanted a QC-led process to resolve the matter.
"I also want to assure myself that appropriate victim support and advocacy have been put in place around the complainants and ensure the terms of reference in the appeal covers the entirety of the process."
She also said she was still happy to meet with the complainants and would work to make that offer available to them if they wished to take it up.
"I want a justice system in New Zealand where people feel comfortable coming forward and are listened to, but I also need to ensure the Labour Party lives up to that expectation too."
A statement has been issued on behalf of six of the victims in the Labour Party controversy, who are alleging behaviour ranging from bullying and intimidation, to sexual assault.
It said they would not be making any media appearances due to the ongoing attempts by the Labour Party to "ascertain their identities".
They said the resignation of Mr Haworth was a step "towards acknowledging that Labour has failed these survivors", but there was still much more work to do.
They were calling on the Prime Minister to make sure there were internal changes to ensure it "never happens again" and to "fundamentally reform" the party's culture.
And they wanted an apology from Mr Haworth "for the handling of this process under his leadership".
They also pointed out there was still a person facing serious allegations, and action had to be taken to ensure no-one else was harmed.
In a statement, Mr Haworth said his resignation was effective immediately.
"I've come to the conclusion that regardless of the outcome of the appeal process into complaints about a Party member, fresh leadership will be required to take forward any recommendations from that process.
"I have greatly enjoyed my time as president and will continue in my lifelong support of Labour and its principles,'' he said.
Mr Haworth said he'd be making no further statements on his decision.