A former Labour Party president is bewildered that Jacinda Ardern was not warned earlier about sexual assault allegations within the party.

Fresh reports of an alleged sexual assault on a 19-year-old member of the party in 2018 surfaced on Monday, with party President Nigel Haworth resigning on Wednesday over his handling of the debacle.

"I was provided some of the correspondence from complainants written to the party several months ago," Ardern said in a statement on Wednesday.

"It confirms that the allegations made were extremely serious, that the process caused complainants additional distress, and that ultimately, in my view, the party was never equipped to appropriately deal with the issue."

Sir Bob Harvey, who used to hold the role of party president, says it's shocking because Haworth worked closely with the Prime Minister.

"I am bewildered about the closeness between the president and the leader of the parliamentary group and that's what the Prime Minister is.

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"There has got to be a very close understanding on a daily basis and it seems that there's been a gap and that gap is, I think, a real problem."

He said there's an expectation for people to respond quickly with a great deal of honesty and integrity, but there should have been time to work out what exactly happened before forcing Haworth out.

"There should have been a period of 'okay let's wait and see exactly what happened, what he did and what he didn't do'. Right now the guillotine has spoken."

Meanwhile, an international relations expert believes the impact to New Zealand's international reputation will be minimal.

Robert Patman doubts the case will harm Jacinda Ardern's character.

"Rather than focus on one incident I think many people will be looking at her record in office over a longer period of time and not just focusing on one or two episodes."

But he thinks we need to wait and see how the situation is resolved before drawing conclusions.

"We do not know the full picture because it seems that the Prime Minister was told that the allegations were not of a sexual nature."

Patman said Ardern may not have been "tenacious enough" when questioning her collleagues.

Newshub.