Gordon Legal and the Rail Tram and Bus Union have filed a class action in the Supreme Court against Emergency and State Services Super (ESSSuper).

Union branch secretary Vik Sharma accused the fund of short-changing some rail workers by failing to include shift penalty allowances when calculating their super.

The action estimates unpaid entitlements could exceed $40 million across a mix of current and former workers who are or were members of the Transport Superannuation Fund.

"We've been fighting this issue for years and it's come to a point now where we have to take this drastic step," Mr Sharma told reporters on Friday.

ESSSuper is the dedicated super fund for emergency services and Victorian government employees, with roughly 127,000 members and more than $36 billion in assets.

Super is calculated by multiplying a person's gross salary and wages by 11 per cent. Overtime and expenses are excluded but some bonuses and allowances are included.

Gordon Legal partner James Naughton said shift penalty allowances were a significant part of transport workers' take-home pay and should have been factored in.

"The way this particular scheme has been set up in the legislation allows ESSS to include these shift penalty loadings as part of the calculation," he said.

"They haven't done it for these transport workers but for other workers that ESSS are responsible for, they have."

Not counting shift penalties meant total super was 20 to 30 per cent lower, costing each worker up to hundreds of thousands of dollars, Mr Naughton added.

Elwyn Gonsalvez, one of the three lead applicants, said not counting his allowances had left him up to $90,000 worse off.

"I worked for over 45 years and to now be forced into legal action so I can retire is devastating," he said.

Mr Naughton said ESSSuper had obligations to protect beneficiaries and the class action was the last straw after the firm first wrote to its board to raise the issue in January 2022.

"Instead of being co-operative, the board tried to put the blame on other parties," he said.

Mr Naughton said the law firm was aware dozens and dozens of workers had been impacted but estimated the figure was much higher.

"We don't exactly know how many people are affected at the moment but we do think it's probably hundreds," he said.

Gordon Legal is urging current or retired Victorian transport workers to check whether they were a member of the transport fund as they could be entitled to compensation.

ESSSuper chief executive Robbie Campo said the fund made contact with the union's representatives on Friday morning to discuss their claim.

"We are hopeful this matter can be resolved through discussions with the union and engagement with Metro Trains - the employer of most of these members," she said.

A Victorian government spokeswoman said the class action was a matter for Metro Trains, the union and ESSSuper.

Metro Trains declined to comment.

Australian Associated Press