A new allegation has surfaced about a sexual assault at a Labour youth camp.

A woman has complained to Labour's general secretary Andrew Kirton about another sexual assault at a different event to the youth camp incident from last month that involved four 16-year-olds.

Kirton says he was approached about the incident today, which is of a similar nature.

"Another incident occurred at an event a few years ago.

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"I've spoken to the person involved, offered them support, and obviously offered to go to the Police if they wish to do that."

National leader Simon Bridges says the Labour Party has left itself open to accusations of a cover-up because of its failure to disclose alleged indecent assaults at a summer camp near Waihi to police or parents.

The former Crown prosecutor said that the description of the incident, in which an apparently drunk 20-year-old put his hand down the pants of three 16-year-olds, would be termed indecent assault.

The police should have been told about it "and by not doing that, the Labour Party opens itself up to allegation of a cover-up here", Bridges told Heather du Plessis-Allan on Newstalk ZB.

He also said that, as a parent, he would have wanted to know so that his son or daughter was getting what they needed to get through it.

Kirton has apologised Prime Minister Jacinda Ardern for the handling of the matter.

Those affected had not been offered counselling until almost a month after the event.

"We could have brought the professional help in earlier, even though we were trying to be led by those involved.

"Also the delay with the communication after two of them got back in touch was not acceptable."

Kirton said he had not offered to stand down. Asked if it was acceptable given Labour's principled stance on sexual harassment issues, Kirton replied: "Well, we could have done better."

Government minister Megan Woods knew about sexual harassment claims at a Young Labour summer camp after one of the victims approached her.

Prime Minister Jacinda Ardern has criticised her party officials for their handling of the incident at the camp last month, which she had spoken at.

She "immediately" called Labour Party general secretary Andrew Kirton to alert him and asked him to contact the person.

She then let the young person know that a party official would be in touch.

"Two hours later she heard from the general secretary that he had been in touch with the young person and the situation was being handled appropriately."