Prime Minister Jacinda Ardern has acknowledged the Labour sexual assault scandal risks hurting New Zealand's reputation on the world stage as she heads abroad.

Photo: RNZ / Dom Thomas

And she's defending her decision to leave the country while there are still questions about Labour's mishandling of serious allegations.

Ms Ardern is in Japan this week for a long-scheduled trip in which she'll meet her counterpart Shinzo Abe and attend the All Blacks' first Rugby World Cup match.

She will then go on to New York for the United Nations General Assembly for several days of high profile speeches and meetings, including an official sit-down with United States President Donald Trump.

Speaking to RNZ ahead of her departure, Ms Ardern said there was never "an optimum time" to be overseas, but it was important for her to promote New Zealand's tourism, trade and investment opportunities.

"My job is, yes, to be the leader of the Labour Party - and I've set in train all of the processes that we need going forward in that regard.

"But I'm also in the role of promoting New Zealand's interests abroad, and that's a very, very important role that I do not take lightly."

Asked whether the ongoing storm had damaged her international brand, Ms Ardern said: "Obviously, this is something that I need to deal with - and I am."

Ms Ardern accepted that her reputation was directly linked to New Zealand's international profile.

"That's why I need to put it [Labour's botched response] right and that's absolutely been my focus," she told RNZ.

The party's handling of the sexual assault allegations has made headlines around the world including in the Washington Post, the New York Times and The Guardian.

During Ms Ardern's address to the UN General Assembly last year, she promoted women's rights and gender equality and referenced the #MeToo movement, saying: "Me Too must become We Too."

Asked whether her message was still credible in light of Labour's debacle, Ms Ardern said: "I absolutely stand by that.

"If you can have something like this happen in the Labour Party, that just demonstrates what work needs to be done."

Ms Ardern is expected to meet a number of world leaders and tech company representatives during her trip to New York.