Little was replaced as leader, and Jacinda-mania swept the country. Ardern put a youthful face on politics, and Young Labour volunteers became the backdrop for many of her campaign events. Commentators started calling it a "youth quake".

When Labour eventually took office in October 2017, the young woman's standing with the party increased and she began to take on more responsibilities. The young man worked in the Labour leader's office, a unit that helps MPs deal with the day-to-day business of Parliament. 

It's based in the Bowen House office block and is separate from the prime minister's office, on the ninth floor of the Beehive. The man was a public servant employed by Parliamentary Service, not the Labour Party.

He resigned on Thursday afternoon, citing stress and a "wish not to be a distraction to the work of the Government". He continues to reject the allegations against him.

Occasionally, the young woman would have to deal with him in the course of their party duties. 

On Valentines Day 2018, she went to his home for an exchange of party documents. Originally, they'd agreed to meet in a public office, but that was closed.

That evening, she says he subjected her to a sexual assault in his home. 

"I tried to pull away but just sort of lay there frozen," she would later reveal. "He pulled down my pants and grabbed me under my shirt. I can recall him pressing down on my thighs. Then over my ribs. 

"I remember struggling, hoping someone would come up the stairs or that some noise would make them come up. And trying to push him off me."

Shocked, and frightened, she did not report the attack to police. But then she began to hear other stories about the man's behaviour. She tried to talk to party officials, one a Young Labour office holder, the other on the party's governing council. No-one seemed to know what to do.

A few weeks after the alleged attack, the party was rocked by allegations of sexual assault at another Young Labour party event. A different man, a 20-year-old, was accused of assaulting four teenagers at the summer camp, held at Waihi, on the Coromandel peninsula. The party came in for fierce criticism over accusations of underage drinking, a lack of supervision and the way it handled the scandal.

Under pressure, party president Nigel Haworth announced a review to be conducted by Wellington barrister Maria Austen. Police began investigating the complaints. (Last week the man admitted two counts of assault at the youth camp, after the Crown withdrew charges of sexual assault. He will be sentenced in early November.)

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Young Labour members were also urged to come forward to Austen with "historical cases" or "other concerns" relating to the party.

The woman decided to contact Austen, and wrote a letter to the review's dedicated email address, spelling out what had happened at the Hamilton marae. Unsure of who would read the email, she chose not to mention the later assault.

Three days later, Austen replied, offering to meet up and asking the woman to provide some screenshots she had mentioned in her email.

But the meeting never happened, because Austen said she wanted to concentrate first on interviews relating to the youth camp assaults.

In August, after an unpleasant encounter with her alleged assailant at a party meeting, the woman decided to contact Haworth.

Her email, on August 6, has the subject line: "I need your help." It is confused, and does not specifically mention either allegation of assault. 

"I'm struggling to be able to cope," she wrote. "[Here she gives the staffer's title] blackmail [sic] people and has shown predatory behaviour. I can't cope - the women can't cope and were so unsure what to do ... I've watched as men into [sic] the party and senior staffers pressure young volunteers into drinking. 

"I've reported preditory [sic] behaviour involving [here she names the man] ... So many of us have come forward and been shut down ... His behaviour has been getting worse over this last year."

A meeting was arranged for the following week, between the woman, Haworth and assistant general secretary Dianna Lacy, at Wellington City Library.

The woman told Stuff that she recounted the serious sexual assault and told Haworth and Lacy of incidents she was aware of involving other women. 

"It was quite formal. I read out some bullet points about what I had heard had happened to other women and what had happened to me …

"Dianna spoke about the values of Ardern and how things might be able to change culturally. Nigel said they took it very seriously and 'we do feel for you'."

Haworth disputes the woman's version of this meeting. In a written statement on Tuesday, he said: "The serious allegation of a sexual assault ... was not provided to the president and acting general secretary at a meeting in the Wellington Central Library." 

Lacy did not respond to requests for comment. 

Later that month, Haworth received Austen's report into the youth camp episode. He refused to make it public but did say the party would implement her recommendations, which included reviewing the party's code of conduct and its policies on alcohol, sexual harassment, bullying and its complaints procedure.

Meanwhile, other women began raising concerns about the staffer's behaviour. Parliament became consumed by the Jami-Lee Ross scandal. The ex-National MP was accused, by four women, of controlling, manipulative and abusive behaviour.

In November, Speaker Trevor Mallard announced an independent investigation - the Francis Review - into bullying and harassment at Parliament.

During this period, there was intense scrutiny on the behaviour of those who worked in and around Parliament and, in the #metoo era, workplace bullying was a live issue.

The Labour staffer attended a Young Labour Christmas party on December 15, and another altercation between him and the young woman took place. One of her friends intervened and says the man threw a punch at him.

Events at that party seemed to have been a catalyst for the complainants, and eventually 12 people raised concerns about the man's conduct.

In February, the party formally agreed to look into the staffer's behaviour. This had to be signed off by its 22-member New Zealand Council. Ardern and deputy leader Kelvin Davis are both on the council, but have not responded to questions about whether they were party to the decision.

Haworth said on Tuesday that the woman's February 2018 allegation of assault was not provided to the investigation. That has been contradicted by at least two complainants and correspondence later provided to Ardern confirmed the complaints were very serious.

Instead of bringing in independent investigators, or asking experts for help, the party appointed three people on its ruling council.

They were Simon Mitchell, a lawyer, Christchurch-based Tracey McLellan, who has previously worked for MPs Ruth Dyson and Megan Woods, and Honey Heemi, of Labour's Māori council.

Interviews took place in March, at Labour's Wellington headquarters, and the staffer instructed a high-profile lawyer. The complainants did not have legal advice, and two have described how one of the panel left the room to take phone calls during their testimony.

The man continued in his job with the Labour leader's office. In late April, some of the women turned up to a social event at Parliament to find he had been appointed as a chaperone.

In June, the investigators reported to the New Zealand Council recommending no disciplinary action be taken. The council accepted that recommendation. Again, Ardern and Davis would not confirm if they were at that meeting or saw the report.

The complainants were later sent "notes" from their interviews with the panel. But they were not transcripts of what was said, and some felt they did not accurately reflect their evidence, or that pertinent details were left out.

Dissatisfied, they asked if there was an avenue of appeal. So, the party asked its solicitor - not an independent adviser - to review the process, but not the allegations.

During that period, some of the young volunteers were asked to stay away from the Bowen House offices where the man worked. Mallard has since said that was inappropriate.

It has been claimed that concerns were raised with Finance Minister Grant Robertson, by one of the complainants, at an event on June 30. He has not responded to questions on this.

By July 12, the complainants had lost patience, decided to go public and sent an anonymous email to several media outlets.

Just over 10 days later, general secretary Andre Anderson wrote to the complainants.

"The email to the media has had the unfortunate effect of increasing the number of people who know something about these matters, which is undermining confidentiality. I think it would be reasonable for you to assume that the content of the email has been circulated to a number of people," he wrote.

"I'm aware that at least one of you has been approached by one or more MPs. But they may only know one of you and the content of the email, rather than all of you."

He then listed "the people who I either know are aware or I've been told are aware". This included Robertson, though Anderson wrote: "I don't know how much Grant was told."

He says that he, or Haworth, knew the following people had been told: Ardern; her former chief of staff Mike Munro; new chief of staff Raj Nahna​; chief press secretary Andrew Campbell; and the party's solicitor Hayden Wilson. "These people only know the basics, including [the man's] identity, but we haven't told them who you are," Anderson wrote.

He then says the man, or a member of his family, had told him four other people knew. These included the man's lawyer Geoff Davenport and E tū senior national industrial officer Paul Tolich, who also sits on the NZ Council. Wellington city councillor Fleur Fitzsimons, and Beth Houston, who works for Cabinet minister Phil Twyford were also listed – both are on the council. "I don't know the extent of their knowledge," Anderson said.

MPs Kiritapu Allen and Paul Eagle are also mentioned: "I don't know the extent of their knowledge," Anderson said. Eagle has since denied he was in the loop.

The first news reports began to appear in early August, and almost all refer to bullying, sexual harassment and sexual assault. On August 6, Ardern spoke to reporters at Parliament and said the party would begin a review.

When asked if Labour had a culture problem, she said she couldn't ignore the fact that complaints had been made.

Ardern has maintained she did not know complaints of serious sexual assault were reported to the party until this week.

"Monday was the first time that I saw details that a complainant had stated that they'd been sexually assaulted and that they'd taken a complaint to the Labour Party. That was the first time," Ardern said at a press conference on Thursday.

She said when media reports first surfaced, five weeks ago, she "sought assurances" from the party and was told "no complainant had come to them and claimed to them they'd been sexually assaulted".

On Wednesday, after speaking with Ardern, Haworth resigned. He stands by his version of events, but won't be interviewed.