The National Party has engaged PwC and Simpson Grierson to carry out its investigation into who leaked leader Simon Bridges' travel expenses information to the media ahead of its public release.

National MP Gerry Brownlee, the Shadow Leader of the House, said the investigation, which began today, would be funded by the National Leader's Office.

"The National Party can confirm that it has commenced an internal investigation into the leak of information about travel expenses.

"There will be no further comment until the investigation has been completed," Brownlee said in a statement.

Simpson Grierson is a well-known law firm while PwC offers services such as forensics and cyber-security.

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Speaker of the House Trevor Mallard last Friday announced he had canned a planned inquiry into the leak, a day after confirming it would be led by Michael Heron QC, and a week after receiving a text from the purported leaker.

The texter, who also sent it to Bridges, said they were a National MP and wanted the inquiry called off on mental health grounds.

Prime Minister Jacinda Ardern yesterday confirmed she spoke to Mallard about the issue on Friday but a spokesman said she played no part in his decision to cancel his inquiry.

Mallard and Ardern have both said they believe the leak was an internal matter for National to deal with.