Parliament will initiate an inquiry into the leak of Simon Bridges' expenses, the Speaker Trevor Mallard announced on Wednesday.
"There is an issue with regard the security of information, and it has the potential to be quite serious," Mr Mallard said.
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"If there's a gap in 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."
Mr Mallard said he will approach the Queen's Counsel to initiate an inquiry, and he expected the inquirer would be assisted by an employment lawyer and a forensic ICT expert who can trace the path of the document.
He said the evidence suggests the information was transmitted electronically.
"My view is that if something was actually forwarded from an MP's machine or a staff member's machine to a news organisation, that shouldn't be too hard to identify."
He warned the leaker that they would be identified by the experts, "unless they have incredible expertise".
"In my opinion someone has deliberately undermined either an individual or the system and I want us, if at all possible, to get to the bottom of it," Mr Mallard said.
"If MPs want to do their jobs properly, they have to trust that the information that is held on their behalf is shared in the way that is agreed."
Mr Mallard said there had been no suggestion that hacking had been involved.
The leak could only have come from the Speaker, Parliamentary staffers or the National Party caucus, all of whom were supplied the information.
The document, leaked to Newshub political editor Tova O'Brien, said Mr Bridges had spent $113,000 taxpayers' money on Crown limos and accommodation during his 'getting to know Simon Bridges' road show.
Mr Bridges has called for someone like a High Court judge to lead an investigation into the leak, saying he's confident it didn't come from within his caucus.