Pressure is growing on Jacinda Ardern to explain when she was made aware of serious sexual assault allegations in her party after the staffer at the centre of the furore resigned.

The individual remains unnamed but is known to have worked in a senior role in Parliament House and had regular interaction with senior Labour ministers.

According to allegations published on the Spinoff website on Monday, the staffer is alleged to have seriously assaulted a 19-year-old Young Labour member at his home early last year. The alleged victim claimed the Labour party ignored her numerous official complaints about the matter.

On Thursday, the man sent a statement to RNZ denying “the serious allegations made against me” but saying he was resigning due to stress and to avoid distracting from the government’s work.

“I am cooperating fully with the Dew inquiry that is now under way and will continue to do so, having been assured that this process will be fair to all parties,” the statement said.

Scepticism is mounting as to when the prime minister found out about the allegations, and Ardern has been accused of letting down survivors who viewed her as a champion for women’s rights and equality.

Ardern has repeatedly said she first became aware of the allegations from reports on Monday, but documents obtained by domestic media show senior Labour figures were sent testimony of the sexual assault allegations last year.

A number of political journalists said the allegations were a critical moment for Ardern’s leadership. Newshub’s Duncan Garner said the idea that the prime minister did not know about the allegations was a “massive stretch”, while Stuff’s Andrea Vance said the prime minister’s insistence that she did not know was “hard to swallow”.

The prime minister faced a barrage of questions over the issue from the press on Thursday as she announced policies on mental health and the school curriculum.

A QC has been appointed to investigate Labour’s handling of the complaint and is due to report back to Ardern within a month.

On Wednesday, the party’s president, Nigel Haworth, resigned after emails showed he had been sent testimony detailing the alleged sexual assault last year, despite claiming to have not been told about it.

There were at least six other complaints against the man relating to misconduct, of which he was cleared by an internal Labour investigation earlier this year.

The allegations are the worst crisis to face Ardern since she took office in 2017, not least for her strong focus on women’s and young people’s rights.

Speaking last year at the UN general assembly, she said the #MeToo movement must become “we too”.

Ardern told the assembly: “It seems surprising that in this modern age we have to recommit ourselves to gender equality, but we do. And I for one will never celebrate the gains we have made for women domestically, while internationally other women and girls experience a lack of the most basic of opportunity and dignity.”