Prime Minister John Key is defending the $250,000 spent so far on an attempt to find out who leaked Ministry of Foreign Affairs and Trade Cabinet papers.
The State Services Commission appointed Paula Rebstock to find out who leaked the restructuring papers to Labour Party MP Phil Goff more than six months ago.
Ms Rebstock has been paid $100,000 so far, with no indication of when the investigation might finish.
Mr Goff said the "witch hunt" is a waste of taxpayers' money and keeps the issue of a botched exercise alive within the ministry. He said the public will be asking what the investigation will achieve.
But John Key said on Friday the leak was part of a significant campaign and the State Services Commissioner wanted the investigation.
The State Services Commission said the investigation requires a thorough approach, including interviews with staff and an audit of all related written and electronic material.
It said due time and consideration was being given to maintain a fair and appropriate process.