Prime Minister Jacinda Ardern and National leader Judith Collins have been swapping strongly worded letters regarding the "totally inappropriate" saga in the House earlier this month.

Collins is still calling for Speaker Trevor Mallard's head, saying Ardern should tell him to leave the role "for the sake of upholding the integrity of Parliament".

Mallard had used the privilege of the House to provide the "truth" about what he had falsely described as rape.

He alleged that the Parliamentary staffer he falsely accused of rape, and apologised to through lawyers, had committed "serious sexual assaults".

In a letter, seen by the Herald, Ardern said that what happened in the House that night "fell well short" of previously agreed behavioural standards.

She made similar comments when speaking to media the day after the saga in the House, calling what happened "totally inappropriate".

"New Zealanders expect a higher standard of behaviour from Members of Parliament than they saw last night," Ardern said in the letter.

She called on all leaders of Parliament's political parties to reconvene the Parliamentary Code of Conduct Steering Group to figure out a way of dealing with "sensitive staff conduct matters" such as sexual assault.

In her letter in response, also seen by the Herald, Collins was scathing of Mallard, and of Ardern for her continued support of him as Speaker.

"The debate on the evening of 4 May was unedifying because of the behaviour and statements of Mr Mallard and his reaction to being question, as my MPs were not only entitled to do, but duty-bound to do," she said.

"You have, as Leader of the Labour Party, the ability to tell Mr Mallard to put Parliament before himself, and leave office.

"You should do it, for the sake of upholding the integrity of Parliament," the letter said.

Collins also took aim at Labour MP Willow-Jean Prime's comments in the House that night.

Prime, who said National's statements in the House were "horrific", characterised National MPs as saying, of the alleged victim: "It really sounds like she asked for it. Her skirt was too short. She was drunk. I didn't like what I heard."

She added "you said it" in response to an uproar from National MPs.

National's shadow leader of the House Chris Bishop strongly pushed back on this, saying National MPs didn't, in fact, say this. Prime was forced to withdraw and apologise for her statement.

In her letter, Collins said Prime's contribution to the debate would have been an "embarrassment" to Ardern.

"It was your MP, Willow-Jean Prime, who outrageously and falsely asserted that Opposition MPs said about a victim."

"My MPs did not say that, did not imply it and would never do that."