Unions representing staff at the Ministry of Foreign Affairs and Trade are taking legal advice over an investigation into leaks of confidential information.
The ministry announced in May it will cut about 80 jobs, following leaks of confidential documents about an initial proposal to slash more than 300 jobs and an outcry from senior diplomats and staff.
An inquiry to get to the bottom of the leaks, led by former Commerce Commission head Paula Rebstock, has demanded MFAT staff appear before it - raising concerns from unions and the Labour Party.
The Public Service Association and the Foreign Service Association say the terms of reference of the investigation are extraordinarily broad.
"Our concern is that (Ms Rebstock) has been given the scope to essentially go on a fishing expedition and trawl through a range of information that isn't relevant to the subject of the inquiry. That is what we believe needs to be questioned," PSA national secretary Brenda Pilott says.
The unions have advised staff not to speak to the inquiry without a representative present.
A spokeswoman for the State Service Commission told Fairfax the probe has power to summon any person "to give evidence, and to produce any papers, documents, records, or things in that person's possession or under that person's control that are relevant to the subject of the inquiry", under the Commission of Inquiry Act.
The inquiry was originally aimed at finding the source of a leak of up to three Cabinet papers, but has since been expanded to cover "all the relevant background facts surrounding the development of the MFAT change programme".
Labour's foreign affairs spokesman Phil Goff - who released the leaked documents - says the "witch hunt" is only causing further damage to staff morale.
"The government is simply making a bad situation worse when they should be working constructively to rebuild the ministry they have so badly damaged," he said.