WELLINGTON (AFP) - Stung by sexual abuse claims, New Zealand's ruling Labour Party ordered a review of its internal procedures on Wednesday (March 14) in the first major crisis to hit Prime Minister Jacinda Ardern's fledgling government.

The centre-left party has been rocked by allegations that four teenagers were groped and sexually harassed at a Young Labour camp last month and officials did little to help them.

Police launched an investigation on Wednesday while Ms Ardern, who took power in October, admitted top officials in her party made mistakes in their handling of the incident.

"That's why the party is taking steps not to ensure it won't happen again," she told reporters.

The allegations centre on a Young Labour summer camp in the North Island retreat of Waihi last month, where it is alleged underage teens threw a boozy late-night party.

The newsroom.co.nz website reported that a 20-year-old man sexually harassed four 16-year-olds. Three of the teens said he put his hand down their pants.

It said they reported the man's behaviour to top Labour officials but no action was taken until three weeks later, when the story was poised to go public.

Among those told of the abuse was Labour general secretary Andrew Kirton, seen as the architect of Ms Ardern's successful "Jacinda-mania" election campaign.

Mr Kirton said this week he did not tell police or the teens' parents because he wanted to protect the victims' privacy.

Ms Ardern, who only learned of the incident this week, denied there was an attempt to cover-up unsavoury and possibly criminal conduct.

"It was handled badly (but) I do not believe there was ever any intent to allow harm to be done," she said, adding that neither Kirton nor Labour president Nigel Haworth would resign over the issue.

Mr Haworth said Wellington lawyer Maria Berryman had been commissioned to conduct an external review of Labour's procedures and culture.

"We're taking this extremely seriously, this is the most comprehensive review in this area that I think any political party has put in place," he told reporters.

"We will make this a party that is absolutely safe for people of any age, of any sex, of any gender, of any orientation." Police confirmed they had launched a sexual assault probe but added "we will not discuss specific investigative steps ... or put a timeframe on the investigation."