The 19-year-old woman alleges the staffer sexually assaulted her at his home in February last year.
Wellington: New Zealand Prime Minister Jacinda Ardern apologised 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 19-year-old woman alleges the staffer sexually assaulted her at his home in February last year and she reported it to Labour Party president Nigel Haworth in October that year.
An internal party investigation recommended no action against the man and Haworth argued as recently as Tuesday that the complainant did not inform him about the seriousness of the allegations.
The woman, who has not been publicly named, disputed this, providing media with emails and documents she said supported her version of events.
Other volunteers also backed the woman and said their complaints to the party were not taken seriously. Ardern said she discussed the woman's documents with Haworth on Wednesday morning, as she tried to address the scandal.
"Whilst he stands by the statements he made on this matter, I believe mistakes were made," she said. Ardern added: "On behalf of the Labour Party, I apologise to the complainants for the way this matter has been dealt with."
In a separate statement, Labour said that Haworth -- who has been credited as an unsung hero of Ardern's electoral victory -- had resigned.
Ardern said she never knew about the nature of the allegations.
"I was informed in the very beginning that the allegations made were not sexual in nature," she said.
"That is obviously directly counter to what is now being reported."
A barrister, Maria Dew, has been appointed to review the Labour Party's handling of the case and is due to report directly to Ardern next month.
Ardern said she was willing to meet with the complainants and would ensure they were receiving appropriate support.
"I want a justice system in New Zealand where people feel comfortable coming forward and are listened to," she said.
"But I also need to ensure the Labour Party lives up to that expectation too." New Zealand's next general election is scheduled for late 2020.