New Zealand Prime Minister Jacinda Ardern apologised on Wednesday for her party’s handling of an alleged sexual assault, as a top ally was forced to resign.

In the most serious scandal Ardern has faced since she took office in late 2017, the centre-left leader admitted “mistakes were made” after a Labour Party volunteer accused a senior party staffer of assault last year.

“Raising an allegation of sexual assault is an incredibly difficult thing to do — for additional distress to be caused through the way these allegations are handled is incredibly distressing,” Ardern said in a statement.

The alleged victim said she reported the 2018 assault to senior Labour party members last year, including president Nigel Haworth, but that they failed to act, and cleared the man of six separate complaints after conducting an internal investigation.

In a statement, Haworth 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 Monday, Ardern said that the staffer in question had been working from home for the past five weeks and that no one in parliament house should feel unsafe, a message reiterated by the speaker of the House, Trevor Mallard.

“I want a justice system in New Zealand where people feel comfortable coming forward and are listened to,” Ardern said.

“But I also need to ensure the Labour Party lives up to that expectation too”, she added.

New Zealand’s next general election is scheduled for late 2020.