All National MPs will be asked to sign a waiver allowing their computers to be forensically searched as the hunt for whoever leaked details of National leader Simon Bridges' expenses to the media.

Parliament's Speaker Trevor Mallard announced today that a Queen's Counsel would be appointed to lead an inquiry, to be paid for by taxpayers, into who might have given the information to Newshub ahead of its public release on Thursday.

Bridges yesterday called for an independent inquiry into the leak. Mallard said today that he had consulted Bridges and they had both agreed there was an issue about the security of an issue which had the potential to be serious.

"If there is a gap in the Parliamentary Service I want it to be fixed and if there is not I think it's appropriate for the person who leaked the documents to be identified."

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He expected that the QC appointed to lead the inquiry would be assisted by an employment lawyer and someone with forensic ICT expertise.

The terms of reference would be agreed between Mallard, the QC and Bridges.

The inquiry would look at who had the document shown in Newshub's report or a more general document. It would also look at who forwarded it to whom, who had access to the data and who accessed it.

Parliamentary Service computers were able to be scrutinised without permission of employees but a waiver would be developed which all National MPs and Mallard himself would be asked to sign to enable access.

"I will ask the Honourable Simon Bridges to ensure that all members of his caucus sign that waiver. I will also sign it myself, although the evidence will show neither myself nor my office received the document in electronic form."

The evidence would show who MPs and parliamentary staff forwarded the document to and who printed it.

"It might well be the fact that the level of expertise that is coming into it causes someone to put their hand up because unless they have incredible expertise they will be identified.

"The security of Members' personal information, until it is made public, is something which I think is very important and in my opinion someone has deliberately undermined either an individual or the system. I want us, if at all possible, to get to the bottom of it."

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Mallard did not put a timeframe on the inquiry for either its start date or its end. Asked if he was able to put a cost on the inquiry, Mallard said that was not possible yet but "the answer to that is too much".

The cost would come out of the Parliamentary Service budget.

He believed it was a good use of taxpayers' money.

"It's my view that Members of Parliament and the public have to have certainty that the information they share with their whips and with the Parliamentary Service is cared for with integrity. That is vital to our system.

"Members of Parliament will not be able to do their jobs properly if they don't trust the system."

National MP Gerry Brownlee, Shadow Leader of the House, told reporters the inquiry would almost certainly flush out the leaker.

"Everybody should have a right to communicate freely with their Member of Parliament and if you can have outside bodies accessing that sort of communication, which often is of a very private and personal nature to that individual constituent, then I think you break down the trust that should exist between Members of Parliament and their constituents.

Brownlee said all National MPs would sign the waiver.