Winston Peters has wasted no time wading into the Labour Party investigation, calling the allegations "unfounded fiction", an "orgy of speculation", and "innuendo".

The NZ First leader's inflammatory comments come as Prime Minister Jacinda Ardern seeks to work with the complainants out of the public glare - but she won't take her deputy to task.

"I've rarely seen such a disgraceful episode of unfounded allegations," Peters said on Tuesday. 

He said it was "led by a woman called Paula Bennett making all sorts of vile allegations by way of innuendo without a fact to back it up".

And New Zealand First MPs were lining up to back him up.

"If you are a victim of criminal wrongdoing, do not go to the opposition - go to the police," Shane Jones, Regional Economic Minister, said.

Tracey Martin, Internal Affairs Minister, added: "[Winston Peters has] got a point - I haven't seen any evidence be produced."

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But complainants have released an email sent to lawyer Simon Mitchell who ran the Labour Party's investigation. The email had a detailed account of sexual assault allegations attached.

Peters says he trusts Mitchell, who claims he was never told by the complainant of any sexual assault allegations.

"A man called Mitchell, who I do trust, says he never saw such a complaint. Now, what do you think of that?" Peters said.

He dismissed the screenshots and evidence which suggest Mitchell was sent testimony outlining sexual assault, telling Newshub: "What you've seen is a bad interpretation of what you think you've seen".

The complainants say they're not going to dignify Peters' comments with a response.  

The Prime Minister said it was "not something I'm going to engage in". She said the Deputy Prime Minister is "able to speak for himself".

Ardern also wouldn't answer whether the man at the centre of the sexual assault allegations - who resigned from Parliament last week - received a pay-out from Parliamentary Service or the Labour leader's office.

The Prime Minister was joined at her caucus meeting on Tuesday morning by Tracey McLellan, who was on the panel with Mitchell that investigated the Labour Party staffer. 

McLellan effectively took over as acting Labour Party President following Nigel Haworth's resignation last week.

Newshub asked McLellan if sexual assault allegations had been raised with her and the investigating panel.

She replied: "Look, the only thing that's on my plate today is making sure that all of those resources are going to be available so that we can properly attend to the inquiries."