WELLINGTON, New Zealand (AP) — New Zealand Prime Minister Jacinda Ardern was facing a difficult test of her leadership Wednesday after her party president resigned over the party's handling of a sexual assault complaint.

Labour Party President Nigel Haworth's resignation came after a 19-year-old party volunteer told online site The Spinoff that last year she was pinned down and violently sexually assaulted by a party staffer.

The woman told the website she didn't want to go to the police. She said she later met with Haworth at a library and told him about the alleged assault and also told a panel investigating various claims against the man.

But on Tuesday, a day before he resigned, Haworth said the woman had never told him or the panel about the alleged assault.

Ardern said the party had made mistakes in its handling of the investigation and wasn't adequately equipped to deal with the allegations.

"I do want to offer an apology on behalf of the Labour Party to those complainants in this case who have gone through a process that I believe has caused them harm," Ardern said.

She said a lawyer who is reviewing the case should sort out questions about whether Haworth and others knew about the assault claims.

But Opposition Leader Simon Bridges said there had been a cover up and it defied belief that Ardern didn't know about the allegations earlier. He said it was inevitable that Haworth would resign.

"He's been caught, the prime minister needed a fall guy here, and he's it," Bridges said. "This won't make it go away because there are so many serious questions around who knew what, when."

The assault allegations do not fit well with the image Ardern has cultivated as an empathetic leader who wants to stamp out bullying and other problematic behavior around the Parliament.

The man at the center of the assault claims continues to work as a Labour Party staff member, although no longer from the Parliament premises. Ardern said she was seeking further advice on his employment status.

Ardern has been praised around the world for her leadership after the March attacks at two Christchurch mosques in which a gunman killed 51 Muslim worshippers. She faces a re-election campaign next year when New Zealanders go to the polls for a general election.