Parliament's Speaker Trevor Mallard says there is potentially a serious issue following the leak of National leader Simon Bridges' expenses to the media.

He will appoint a Queen's Counsel to look into the leak, with the terms of reference for the inquiry to be determined.

Mallard said it could be an employment matter, and said the inquiry would look at who provided the information to whom. A forensic ICT person would be involved in the inquiry.

A waiver was being developed which Mallard would himself sign and would ask Bridges to have all his MPs sign to ensure access to their computers.

No start or end date had been put on the inquiry.

"Unless they have incredible expertise they will be identified," he said.

Mallard said he believed someone had deliberately undermined the system, and the security of MPs was important.

He did believe it was a good use of taxpayer' money. "Members of Parliament will not be able to do their jobs properly.'

Mallard said there was no suggestion that journalists' systems would be accessed, the inquiry would look at who released the information.

"I think this is a matter of computer access."

Mallard said if the culprit turned out to be an MP, that was a matter for the party leader but the person's name would be published.

Mallard said it did not look like an error, as it had been done twice.

The Parliamentary Service was required to be politically neutral, he said.

"All evidence on this is that it was transmitted electronically."

Mallard did not know how much the inquiry was going to cost. "The answer to that is too much."

The cost would come out of the Parliamentary Service budget.