Security guards that patrol Melbourne's train stations and depots claim they are being underpaid for risky and dangerous work.

Key points: Workers claim they are being paid below the award wage

Workers claim they are being paid below the award wage Guards told the ABC they were docked $500 pay if vandals graffitied trains

Guards told the ABC they were docked $500 pay if vandals graffitied trains Metro Trains says its has launched an investigation into the claims

It is the second time a Metro Trains contractor has been accused of underpaying its mostly migrant workers.

Metro Trains confirmed to the ABC it was now investigating security contractor Risk Protection Group (RPG).

One of the guards who says he has been underpaid is Pakistani immigrant 32-year-old Qaisir Kahn, who works overnight shifts.

"[It's] quite dangerous. You don't know who's coming. You can expect anything overnight," Mr Kahn said.

He said while working at night in 2013 three intruders at a train depot tipped petrol on him and threatened to set him on fire.

"[They said] to open the gate for them so they can do the graffiti," he said.

Mr Kahn claimed guards were also being docked up to $500 pay if a train was graffitied on their watch.

Qaisir Kahn says he is owed more than $60,000 by his former employer. ( ABC News: Sam Clark )

Mr Kahn said when he commenced work as a train guard in 2010 he was paid a flat rate of $18 an hour, which had since increased to $22 an hour, and he did not receive penalties for weekend work or superannuation.

The award wage for a casual security guard working nights is about $30 an hour.

He approached the Fair Work Ombudsman regarding his rates of pay, but RPG refused to recognise him as an employee.

In a hearing, RPG told the Fair Work Commission Mr Kahn had never been an employee, and in fact was employed by AIG Security Services (AIG), a company it sub-contracted work to.

But the Fair Work Commission rejected that, saying he was a casual employee at RPG.

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"I am satisfied that Mr Kahn was employed by RPG," Commissioner Julius Roe said on February 13.

But the commissioner rejected Mr Kahn's unfair dismissal claim, which was based on his shifts being reduced and relocated after he raised concerns about his pay.

I was punished for speaking out: Kahn

Mr Kahn said after he raised the issue of underpayment, RPG began to monitor him at work, by which stage he had been moved to another RPG sub-contractor.

He said he was sent a video, which was secretly filmed, showing him sitting down at work at 4:00am and checking his phone.

Sorry, this video has expired Qaisir Kahn was secretly filmed while sitting down and checking his phone at work

In the video he is confronted by a supervisor and questioned why he is sitting down.

Mr Kahn then becomes angry and throws an electronic wand which guards use in their work.

Mr Kahn was sacked by the RPG contractor, which cited his "aggressive behaviour" and the damage he had caused to the equipment.

Mr Kahn said he believed he was punished for speaking out about rates of pay but he did not regret, saying somebody had to.

RPG declined to answer questions its guards' pay and conditions.

It declined to answer whether it docked guards $500 pay if a train was graffitied.

In a statement, the company said it disputed the Fair Work Commission's findings that Mr Kahn was an RPG employee.

"RPG maintains its position that it is not (and has never been) Mr Kahn's employer. RPG relies upon both direct employment and third party labour hire in order to provide its clients with exceptional service."

"RPG remains confident that it has complied with all of its relevant obligations."

The company has 21 days to appeal against the commission's findings.

RPG contract could be terminated: Metro

In a statement to the ABC, Metro Trains spokeswoman Sammie Black said an investigation had been launched.

"We take these matters seriously and have commenced an investigation with RPG which includes companies subcontracted by RPG to supply labour," she said.

"Once we have concluded our investigation, we will take appropriate steps to rectify any non-compliance which may include termination of the contract."

In 2015 the ABC revealed Metro was investigating another contractor, cleaning company Transclean, after some members of its largely migrant workforce came forward with underpayment claims.

Rail operator Metro is auditing Transclean, court documents show. ( ABC News )

Ms Black said an independent auditor reviewed Transclean's conduct and the company agreed to regular, ongoing audits.

"Transclean's existing contracts with Metro will expire over the coming months. We continually audit and review our supply arrangements to ensure best practice is delivered," Ms Black said.

Mr Kahn's industrial representative, Garry Dircks, is representing four other RPG security guards with similar claims to Qaisir Kahn's.

"They are a vulnerable part of the workforce, mostly migrant workers, mostly from Pakistan and Indian," Mr Dircks said.

"There is an underclass and this is one of the manifestations of it. It is also a problem when it's effectively happening in the public sector … this is big business providing services to the government. It just should not happen."

Victoria's Transport Minister Jacinta Allan said would follow up claims private security guards were being docked pay for graffiti.

"Look this is primarily a matter for Metro, but the Government across all of its agencies and indeed in any business would expect workers to be paid fairly for a fair days work," she said.

The ABC spoke to a guard working night shift at a suburban train depot who confirmed RPG pay rates, and that guards are docked $500 if a train is graffitied.