Labour "let down" Prime Minister Jacinda Ardern by not telling her about allegations of sexual assault at one of the party's youth camps, former prime minister Helen Clark says.

Ardern was this month blindsided when asked by media about claims four teens were sexually harassed or assaulted by a 20-year-old man during a Young Labour camp near Waihi in February.

It was later revealed the party's president, Nigel Haworth, its general secretary, Andrew Kirton, and later senior cabinet minister Megan Woods knew about the allegations, but that no one told the prime minister.

Clark has now told Stuff that it was "unbelievable" Ardern had been kept out of the loop.

"Jacinda was let down. She should have been told. Immediately, actually. And then events would have taken a different course," Ms Clark said.

"And I cannot understand why she was not told. Unbelievable."

Asked what she would have done if she had not been told, the former prime minister said: "If you get out the book and ask 'what would Helen have done?' ... draw your own conclusions."

"And she's done her best to repair it after the event, but leaders never like to be blindsided."

Ardern this month defended being kept in the dark, saying it was for the good of the victims for the circle of people involved to be kept small.

"This is not about me and political management. It's about supporting the young people involved," she told reporters.

- NZ Newswire