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.