National's MPs have signed waivers giving investigators access to their electronic devices as far back as February as the party hunts for the person who leaked details of leader Simon Bridges' expenses to the media.

The leak, of Bridges' travel and accommodation expenses for the June quarter, were reported by media three days before they were publicly released on August 16. But Bridges confirmed today that MPs were asked to sign waivers that would allow investigators to go through their mobile devices and computers all the way back to February.

The inquiry, being conducted by PwC and Simpson Grierson, will look at traffic from National MPs' and staff computers.

"I've signed the waiver myself. From memory it's February. If we're going to do this, we want to do it well. To do that and to give PwC here the best chance of doing a thorough job you do want a time period to do it," Bridges told reporters today.

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He gave an assurance to his MPs that the access would not be used as a "fishing expedition" for other information.

"Absolutely, categorically.

"There is no ulterior thing that will happen here."

All MPs have now signed the privacy waiver.

National's deputy leader Paula Bennett and shadow leader of the House Gerry Brownlee are overseeing the investigation.

Today's National caucus is the first time it has met since Speaker Trevor Mallard overturned his decision to hold an inquiry into who might have leaked the travel expenses to Newshub.

On August 23 Mallard named Michael Heron QC to conduct the investigation.

But he cancelled it on August 24 after it was publicly revealed that Newshub, Mallard and Bridges had received an anonymous text from someone claiming to be a National MP and pleading for the inquiry to be called off for the sake of his or her mental health.

Bridges was not entirely convinced it was from a National MP but Mallard said the text was from someone who was clearly very disturbed and publicity around the text would make that worse whether the person was an MP or a staffer.

The police know who the leaker is but are refusing to say. Bridges referred the text to the police and they traced the person out of concern for their welfare.

Brownlee would not guarantee that the result of the inquiry would be made public and hinted that if the culprit was identified and had genuine mental health issues that it might not be.

"That will entirely depend on the circumstances," he said yesterday.