The president of New Zealand’s Labour party has resigned over the handling of sexual assault allegations against a party staffer.

Jacinda Ardern announced Nigel Haworth’s resignation on Wednesday, two days after details of an alleged assault against a 19-year-old were published by the Spinoff. As party president, Haworth oversaw the investigation of the complaint and several others levelled at the staffer by a string of Labour members.

“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. She said correspondence from complainants had confirmed the allegations “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”.

She said Howarth had stood by his previous statements but she added: “I believe mistakes were made.”

Ardern apologised to the alleged victims, and extended an offer to meet. “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.

The alleged victim told the Spinoff she reported the 2018 assault to senior Labour party members last year, including Haworth, but that they failed to act, and cleared the man of six separate complaints after conducting an internal investigation. She said inaction by senior party figures had left her feeling “angry, quite fearful and desperate”.

She is reportedly one of seven people to have complained about the Labour staffer. The staffer was cleared of any wrongdoing by an internal Labour investigation, and has faced no disciplinary action.

Ardern said this week that the media report was the first time she had heard that the allegation was of a sexual nature. “I’m obviously incredibly frustrated, disappointed and concerned about this situation and the competing advice I have received,” she said.

The prime minister said a QC would investigate how the party responded to the allegations of misconduct and sexual assault.

New Zealand Labour party president Nigel Haworth. Photograph: Historic Collection/Alamy Stock Photo

Haworth earlier denied the alleged victim’s account, and said neither he nor the acting general secretary, Dianna Lacy, were ever told of a sexual assault allegation against the staffer.

On Wednesday, Ardern 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 have made it clear that I want the QC-led appeal process to resolve this matter,” she said. “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.

This is not the first time the Labour party has been accused of negligence regarding accusations of sexual assault. At a young Labour summer camp in 2018, a 21-year-old attendee was charged with groping and inappropriately touching and kissing four other attendees after excessive drinking. The case is before the courts.

Australian Associated Press contributed to this report