The State Government has revealed it regularly uses private investigators to spy on public servants accused of misconduct and wrongdoing.

In answers to parliamentary questions by the State Opposition Leader, the Government revealed its agencies and trading enterprises spent just over $1 million on private investigators during 2014-15.

On one occasion, external investigators were used to investigate a worker's substandard performance.

The most prolific users of private investigators were WA Health ($212,659), Horizon Power ($156,357), the Disability Services Commission ($150,610), Water Corporation ($118,300) and the Department of Transport ($89,573).

Other notable spenders included Main Roads ($73,548), the Metropolitan Redevelopment Authority ($43,531), the Department of Corrective Services ($30,074), and the Housing Authority ($26,245).

Reasons given for using private investigators included staff misconduct, breaches of discipline, workplace bullying and investigating the leaking of sensitive and confidential information.

Main Roads Western Australia called in private investigators over "substandard performance".

Opposition Leader Mark McGowan said the Government should not be spending money on private investigators when it has other investigative agencies at its disposal, including the Corruption and Crime Commission (CCC), which has an annual budget of $33 million.

The Public Sector Commission also deals with minor misconduct.

"It seems to me to be overkill to spend that sort of money," Mr McGowan said.

"It's exorbitant, it's wasteful, it's the sort of thing that makes people wonder why the Government is spending money on this rather than the things that ordinary people care about."

Workers would feel trust had been breached: union

The Community and Public Sector Union said its members would react poorly to the news.

"I think they would feel very much that their trust had been breached," the union's Rikki Hendon said.

"It's hard to see how the Government can justify the use of private investigators, especially when the CCC, the Public Sector Commission and HR units are already set up to ensure employee conduct in the public service is appropriate," she said.

"Substandard performance is really the realm of human resource officers and managers in the public sector.

"It seems very, very unusual that somebody would be investigated by a private investigator about their work performance."

Mr McGowan called on Premier Colin Barnett to end the practice of using private investigators.

"The Premier has the capacity to stop this sort of thing happening if he wants to," he said.

Mr Barnett was unavailable to respond.

The Public Sector Commission issued a statement, saying: "These arrangements have been in place for successive governments and in most cases the relevant unions support independent assessments of these matters."

It said the external investigators had a good understanding of the legislative environment and investigative processes appropriate to the public sector.