While she said she was “not bothered” that she had not been informed, she told Radio New Zealand she was concerned about “a delay in services and support being offered to those involved.”
Ms. Ardern said that the man who had been accused was ejected from the camp the morning after the party and that the 16-year-olds were “asked what they’d like done.”
But she said Labour officials now acknowledged that “we should have brought in the professionals straight away.”
Ms. Ardern said that the 20-year-old was not a Young Labour member or part of the staff and that she did know why he was there.
The Labour Party’s general secretary, Andrew Kirton, defended on Monday his decision not to involve the police or the victims’ parents, saying it had been a “survivor-led response” rather than a cover-up, which permitted the teenagers to decide whether they wanted the matter taken further.
A sexual abuse support agency was asked to advise on the matter, but not until March 5, three weeks after the event. By that point, one of the victims had complained to a senior Labour lawmaker about the party’s lack of response.
Conor Twyford, chief executive of the agency HELP, said that while it was the first time she had been formally approached, it was “not at all” the first time she had heard of sexual misconduct at political party events.
Ms. Twyford supported Labour’s decision to contain the circle of people who knew about the assaults, but she criticized officials for their delayed response.
A lawmaker from the opposition center-right National Party, Judith Collins, criticized the decision not to involve the police.
Mrs. Collins, previously a lawyer, said that “a culture of not dealing with sexual assaults seriously and as the criminal acts that they are has fostered a culture that makes it very difficult for victims to come forward.”
But said she did not think sexual misconduct was a particularly political problem.
The youth wings of New Zealand’s other major political parties said they had not had similar issues. But the sole lawmaker from the libertarian party ACT, David Seymour, said his group had stopped running youth camps.
Mr. Seymour said that there had been no particular problems but that he was concerned that laws against underage drinking made the party liable if anything went wrong.
The police on Tuesday encouraged complainants to contact them and said an investigation could begin regardless of whether a complaint was made.
Mr. Kirton, the Labour general secretary, said there would be “an external review of our policies and procedures including those involving alcohol.”
The Labour Party came under similar criticism in 2017 when it was found to have used 80 unpaid, foreign interns to help it campaign. Some of the interns complained about substandard accommodations and the tasks they were asked to do, which included canvassing and door-knocking.