National Party leader Simon Bridges has lashed out at Parliament Speaker Trevor Mallard for cancelling an inquiry into the travel-expenses leak 24 hours after confirming it was going ahead, and suggested he had been influenced by Prime Minister Jacinda Ardern.

He said if Mallard or Ardern had any new information, they had a duty to share it with National.

If the issue remained unresolved, National blamed Mallard, Bridges said.

On Thursday last week Mallard named Michael Heron QC to conduct the inquiry into the leak.

On Friday afternoon Mallard cancelled the inquiry – more than a week after the alleged leaker sent a text pleading not to hold the inquiry.

"Nothing had changed fundamentally on the Friday other than that the Prime Minister said it was an internal matter for the National Party," Bridges said.

"Surprise, surprise, Trevor Mallard then changed his position.

"I know of nothing that gives any good reason for his change unless the Prime Minister or he knows something we don't and if they do, they should be sharing it with the National Party," he said.

"I believe he is obliged as Parliament's Speaker, not a partisan one, to tell us what he knows unless there is an exceptional reason not to."

The criticism of Mallard is in itself a new twist in the saga that is likely to see Bridges at the very least forced to apologise to Mallard if not referred to the privileges committee.

The saga began on Monday, August 13 when Newshub ran a story about Bridges' latest travel expenses which were abnormally high because he had just completed 70 well-advertised public meetings around the country as the new Leader of the Opposition.

Mallard agreed on August 15 to hold an inquiry but on August 16 Mallard, Bridges and Newshub received an anonymous text from someone claiming to be the leaker and pleading for the inquiry not to go ahead on the basis of their mental health problems, saying they were a National MP, with references to some things that had happened in caucus.

Bridges contacted the police on August 17 with concerns for the texter's welfare. The police have identified the texter but won't tell Bridges who it is.

Mallard appointed Heron to conduct the inquiry, despite having received the text the previous week.

The existence of that text was not revealed until last Friday – and later that day Mallard issued a statement cancelling the inquiry.

The statement said: "The text is from someone who is clearly very disturbed and today's publicity will almost certainly make that worse."

Mallard said the person who sent the text was the leaker. "He or she has details of events that it is unlikely anyone outside the National Party would be privy to."

Bridges told the Herald National was preparing its own investigation and would seek consent of MPs and staff to look at their computer. That had not yet happened.

"National will have some form of investigation to do our best to get to the bottom of this and work out if it is a National MP or staffer.

"The problem is it may not be a Nat and therefore we won't get to the bottom of it and National blames Trevor Mallard for that.

If the leaker was a National MP or staff member, Bridges encouraged them to come forward.

"If it is from within my team, I certainly encourage them to come forward. We will be incredibly understanding about their health and wellbeing and what is best for them. I promise them that."

He could not guarantee confidentially because he did not know what he was dealing with.

"I can assure the person that if they present to me or indeed someone else that they trust around the National Party, we will treat their health and wellbeing of the utmost seriousness and we will want to be understanding rather than punitive."

Speaking on Friday, Ardern said the inquiry should be stopped if it was proven the individual had mental health issues, and it was an internal matter for National.

"I would want to deal with that internally but that is a matter for the leader of the National Party."

"If indeed this is an issue that's come out of the caucus, and if there are indeed mental health issues, it would strike me it needs to be dealt with really sensitively. It is perhaps best dealt with internally than externally."

Timeline

August 13 - Newshub publish story based on Simon Bridges' leaked expenses.
August 15 – Speaker Mallard agrees to hold inquiry.
August 16 – Bridges, Mallard and Newshub receive anonymous text message allegedly from National MP pleading for inquiry to be called off on mental health grounds.
August 17 - Bridges talks to mental health experts and tells police about text on advice.
August 19 – Police tell Bridges they have identified and contacted texter (won't name them) and that the person is getting support. 
August 23 - Mallard names Michael Heron QC to conduct inquiry.
August 24 – RNZ reveals texts were sent previous week to Bridges and Mallard; Ardern and others comment publicly.
August 24 - Mallard cancels inquiry.