Jacinda Ardern knew about Labour staffer sexual assault allegations - Simon Bridges

National Party leader Simon Bridges believes Prime Minister Jacinda Ardern knew about sexual assault allegations against a staffer before this week as she claims.

Detailed sexual assault allegations against a party staffer from a 19-year-old Labour Party volunteer surfaced on Monday.

Newshub first broke the story about multiple complainants coming forward with allegations of bullying and sexual harassment against the staffer in August.

But Ardern said she didn't know about the sexual assault claims until Monday and had been told by her party the complaints hadn't been sexual in nature.

However, Newshub has also obtained emails that show Labour was sent details six months ago about the sexual assault allegations. A complainant also says she provided the claim to a panel investigating complaints. The party denies it knew about the claims.

Bridges told The AM Show on Wednesday that he didn't believe Ardern's story rang true.

"I think she knew. All very well, it may be that the President [Nigel Haworth] is in the wrong, but when you think about this, the Prime Minister, if you stand back and look at it, is ducking and diving accountability," he said.

"You have this being in the media and indeed her being interviewed about alleged sexual assaults for several weeks now... I think she knew and she is saying she didn't."

Newshub revealed in August Finance Minister Grant Robertson was aware of the investigation and some complaints, but he's refusing to say how much he knew. 

"I am not going to comment any further than what I have on that because I will be undermining the privacy," he told Newshub. 

Bridges said Robertson looked "dodgy".

"Everyone else seemed to know but [Ardern]. Now that is possible, I just think it is very, very unlikely."

After Newshub's August story, an investigation into the complaints process was launched. That is being headed by Maria Dew QC. Ardern is wanting to wait for that report before taking further action, but she has said she expects Haworth's resignation if he is found to have done anything wrong. 

"I think if the findings are that he hasn't managed it properly, I think he would have his own expectation on himself," she told The AM Show on Tuesday.

"But he knows that I'll be dealing with it decisively once I have that report back."

On Tuesday a group calling themselves "Me Too Labour" released an open letter to Ardern calling for an apology and resignation from Haworth.

Newshub.