But an Endeavour Energy spokesman said it was the "legitimate costs of doing business".

Figures obtained by the Electrical Trades Union (ETU) and the United Services Union (USU) under a freedom of information request, show that Endeavour spent $48,149 from January 1 to October 10 last year on private investigators and investigation firms.

ETU Wollongong organiser Dave McKinley said some of that money was spent tailing Illawarra employees.

"I do know some of the guys were aware someone was following them from time to time," he said.

"From what I understand this was nothing to do with any workers' comp. This was based purely on their disciplinary policy and trying to dig up dirt on various members."

Mr McKinley said the investigations related to issues such as the use of company vehicles and other "fairly minor matters in our eyes".

He said a report on the investigation would be handed in and then, on occasion, attempts would be made to sack the worker under investigation.

Mr McKinley said the union was not impressed Endeavour Energy was investigating its employees.

"Given the fact that it's a government operation, it's fairly draconian," he said.

"It's not something you'd expect in private industry, let alone a public company."

As well as spending money on private eyes, the FOI request also uncovered figures that show Endeavour Energy spent more than $1.5 million on hiring legal firms between 2010 and 2014. Of this, $1.42 million was spent on industrial matters and $150,000 on disciplinary cases.

Mr McKinley said a sizeable chunk of the $1.42 million was from Endeavour's repeated legal failures to overturn a Fair Work Commission ruling against urine testing employees for drugs and alcohol in preference for mouth swabs.

He said the company would use lawyers when dealing with the unions.

"Every time we go to the industrial relations commission, they turn up with barristers.

"The commission was set up so you wouldn't need to have an overdose of legal fees but unfortunately Endeavour continually brings barristers to the Fair Work Commission," he said.

Mr McKinley said this spending was particularly galling at a time when Endeavour was claiming job cuts were needed to save money.

"It's a complete waste of taxpayers' money," he said.

The Endeavour Energy spokesman said the investigations were related to the company code of conduct.

"Unfortunately, claims about the conduct of staff are made from time to time that must be properly investigated," the spokesman said.

"In some cases, this may involve an external investigator.

"These are legitimate costs of doing business.

"Recent matters raised before the ICAC only serve to remind us that action must be taken when claims are made."

Other energy networks had similar spending - Ausgrid spent $1.45 million on legal costs and $40,000 on private investigators.

Businesses hiring private investigators to spy on employees is becoming more common in the Illawarra, according to a Wollongong lawyer.

Michelle Walsh, an employment law specialist and a partner with Turner Freeman Lawyers, said it was unlikely Endeavour Energy was the only company in the Illawarra hiring private eyes or using specialist legal representation.

Ms Walsh said anecdotal evidence suggested many public and private sector organisations were increasing their use of specialist legal and investigative services.

“While Endeavour Energy’s spend of $1.6million on these services appears to be high, there is anecdotal evidence that major employers in the Illawarra are increasingly utilising legal firms and even private investigators as part of their management of workplace disciplinary matters,” she said.

“One advantage of employers using external lawyers is that legal privilege may apply, which means the company can avoid providing the results of their investigations or advice to the employee involved, a relevant union, or even a court or industrial tribunal.”