"What I have seen raises serious questions about the nature of engagement between Thompson and Clark and state sector agencies," Hughes said in a statement.

"As a result I have decided to widen the terms of reference to cover all state services agencies.

"I need assurance that the use of, or contact with, external security consultants by government agencies is consistent with the State Services Code of Conduct."

Hughes appointed Doug Martin in March to investigate the use of Thompson and Clark and other security firms by Southern Response.

Martin is looking into whether the Southern Response agency, set up to settle quake insurance claims, spied on claimants.

That inquiry was expanded shortly afterwards to include the Ministry of Business, Innovation and Employment's (MBIE's) relationship with Thompson and Clark. Activist groups say they have been spied on by Thompson and Clark for MBIE.

The Ministry of Foreign Affairs and the Department of Conservation have also used Thompson and Clark in the past, RNZ has reported.

Hughes has also appointed high-profile Auckland barrister Simon Mount QC to the investigation team.

Prime Minister Jacinda Ardern has described Southern Response's use of Thompson and Clark as "inappropriate" and warned other government departments not to use them in the same way.