The State Services Commission has appointed Paula Rebstock to lead an investigation into the leak of Ministry of Foreign Affairs and Trade Cabinet papers last week.
The papers were leaked to Labour MP Phil Goff and related to restructuring plans at MFAT.
Mr Goff says the documents confirm the Government will do a U-turn on its original planned cuts for the ministry because it failed to get staff buy-in.
The commission says the papers were provided in confidence to senior ministry officials, the Department of the Prime Minister and Cabinet, the State Services Commission and the Treasury.
MFAT chief executive John Allen supports the investigation and State Services Commissioner Iain Rennie says the findings will be made public as soon as possible.
Ms Rebstock is the former chair of the Commerce Commission.
Prime Minister John Key says he is deeply disappointed by the leaks and is sceptical the source will be found.
"Well, I have confidence in the people we've been talking to. Where the leak came from, I don't know - maybe they'll be able to discover that.
"But I've been very historically sceptical where there's been in the time I've been Prime Minister quite a number of investigations in this regard and they've never found the successful answer yet."
However, Phil Goff says the investigation is a waste of taxpayer money.
"They've already spent this year $9.2 million on expensive consultants who have come up with a plan that even the minister himself believes now is no longer workable.
"So what to they do? They take on another highly paid consultant to launch a witch-hunt that won't come up with any answers."
Mr Goff says MFAT's restructure has been botched from the beginning.