The figures, obtained by the Electrical Trades Union and the United Services Union under freedom, show that the three largest state-run providers – Endeavour Energy, AusGrid and Essential Energy – spent $4,604,703 on external legal services alone between 2010 and 2014.

The majority of this – $4,153,837 – was spent on "industrial matters" while the remainder was spent on investigations into disciplinary matters, including $389,387 on private investigators.

Networks NSW, which represents the three electricity providers, said it was necessary to hire external investigators to investigate code of conduct breaches and pre-employment checks, describing them as a "legitimate cost of business".

But the unions said it was outrageous that the providers were using public money from electricity consumers to pay for attacks on workers, including spying on them.

"Networks NSW have been crying poor in recent months, announcing that 2800 jobs at Ausgrid, Essential Energy and Endeavour Energy need to be slashed," Electrical Trades Union secretary Steve Butler said.

"What they haven't revealed is that they were able to find more than a million dollars a year to spend on external law firms and private investigators solely to attack the hard-working men and women who maintain our electricity network and respond during natural disasters."

"Transgrid, who announced no job losses following the determination of the Australian Energy Regulator, has also been shown to have far more modest spending habits compared to the remaining three electricity network companies, who are currently slashing jobs."

Industrial law expert Michael Doherty said the use of external lawyers and private investigators was increasing in both the public and private sector but that $4.8 million was "a considerable amount for anyone to spend on those services" in four years.

"The amount companies are spending on external lawyers is increasing, particularly for disciplinary matters," Mr Doherty, from Maurice Blackburn lawyers, said.

"One of the reasons for that is that they can then use legal privilege to avoid providing the results of the investigation to the employee or the court or tribunal involved."

He said private investigators were commonly used to investigate worker's compensation claims, such as those stemming from a workplace injury, or more generally to catch people out.

"In one case, the company had an investigator waiting to catch staff going over the road to the pub, so that it could breath test them when they went back. It's happening more and more often."