Southern Response Board chairman Ross Butler said the contractor was picked in early 2014 in response to an escalating level of threatening and aggressive behaviour from customers towards staff and senior management.
"Given the environment the company was operating in at the time, and conscious of its obligations to ensure the safety of its staff, Southern Response sought external assistance for a security review," he said.
"The contractor was asked to provide independent security advice, and assess the risk to safety of Southern Response employees and directors."
The safety and wellbeing of staff and customers was of prime importance, he said.
"Our staff have a right to be able to interact in an environment safe from threats or harm, and the board has taken zero tolerance stance in respects of threats and hostile behaviour towards them."
Butler said it planned to cooperate fully with the State Services Commission inquiry.
Thompson & Clark director Gavin Clark said in a statement it would not reveal who it worked with. He did not respond when claims were put to him that the company had spied on insurance claimants for Southern Response.
"Thompson & Clark do not disclose our clients nor operations, we can however advise that as licensed private investigators we operate within the law and in compliance with industry standards and guidelines.
"We have nothing further to add and I trust that this satisfies your request."
Thompson & Clark has previously been accused of spying on environmentalists.
Greenpeace filed a civil suit against the company last year alleging it had caught spies in the act of tracking its staff and supporters and compiling detailed dossiers.
And in 2007 the Government reprimanded Solid Energy for using paid informants.
The company was accused of hiring Thompson and Clark Investigations which paid a 25-year-old Canterbury University student about $400 a month to infiltrate environment group Save Happy Valley.
