The close relationship between private investigators and some parts of the public service is concerning and needs to be investigated, State Services Minister Chris Hipkins says.

"The use of private investigators and the very close relationships they seem to have had in the past with some parts of the public service is very concerning and we need to get to the bottom of that," Hipkins said this evening.

State Services Commissioner Peter Hughes today announced that an inquiry into the use by state sector agencies of Auckland-based private investigation firm Thompson and Clark had been widened to include all government agencies.

Hipkins welcomed the move.

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"There are some historical matters that have come to light and the commissioner has come to us with a recommendation to extend the breadth of his investigation across the whole public service.

"The Prime Minister has been clear that this Government does not want departments using Thompson and Clark for the purposes of spying," he said.

Two agencies, the New Zealand Security Intelligence Service (SIS) and the Ministry for Primary Industries, today said they had opened investigations of their own after information came to light during inquiries into the use of Thompson and Clark.

Director-General of Security Rebecca Kitteridge launched a probe into the conduct of some SIS staff after correspondence raised concern about favourable bias towards Thompson and Clark.

In a letter addressing an Official Information Act response, Kitteridge said she expected high standards of professionalism and conduct from all SIS staff.

"The tone of some of the being emails to you raise concerns about the standards of professionalism being displayed," Kitteridge wrote.

"In light of this correspondence, I have asked for several matters to be looked into. The emails raise questions in relation to conduct and possible bias in favour of Thompson and Clark. These questions are the subject of an internal investigation.

The emails show Thompson and Clark calling on the SIS Protective Security Requirements team to help present information to companies including Otakaro (formerly Cera) and government insurer Southern Response.

The Ministry for Primary Industries (MPI) said it had uncovered evidence of potential serious staff misconduct involving Thompson and Clark.

"The misconduct was uncovered as part of the preparation of a response to an Official Information Act request for all correspondence between MPI employees and Thompson & Clark Investigations Ltd (TCIL)," MPI said in a statement.

MPI said the misconduct related to events that occurred some years ago and involved several employees who no longer worked for MPI.

"We are working to assist the SSC inquiry and are conducting our own internal inquiry," acting director-general Bryan Wilson said.

Hughes said it was the material from Official Information Act requests to MPI and SIS that persuaded him to expand his inquiry.

"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.

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

In March Hughes appointed former State Services Commissioner Doug Martin to investigate the use of Thompson and Clark by Southern Response.

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

That inquiry was expanded shortly afterwards to include 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 Trade and the Department of Conservation have also used Thompson and Clark in the past.

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