Senior 7-Eleven management have apologised "unreservedly" to exploited workers and vowed to fully refund underpaid staff, even if they are now overseas.

A Senate Committee held a special public hearing in Melbourne to examine the exploitation of workers at the chain store, as part of an inquiry into Australia's temporary work visa program.

The convenience stores have come under scrutiny following an ABC Four Corners investigation which revealed the company was systematically paying its workers about half the minimum wage.

Many of the staff were foreigners who were being forced to work in contravention of their visa conditions.

The inquiry today heard the underpaying of staff in the convenience store chain was systemic and had been happening for decades.

7-Eleven Australian chairman Russell Withers admitted the behaviour was abhorrent.

"I want to stress that this has been highly embarrassing and I apologise unreservedly to any worker that has worked in a 7-Eleven store who has not been paid correctly," he said.

New methods of staff exploitation were revealed during the inquiry, including some franchise owners who charged workers tens of thousands of dollars to secure a working visa.

Mohammed Rashid Ullat Thodi lost his job as a result of speaking out about the pay scam.

He told the inquiry some store owners had charged workers $30,000 to $70,000 to sponsor them on a visa.

"While it could be either an arrangement of taking the money off their pay, like if you work this many hours you could get this many pay," he said.

He said the money went straight to the franchise owners.

Mr Withers maintained rogue franchisees were the cause of the problem.

"It is simply not in our knowledge whether the franchisee employee has been underpaid or not," he told the inquiry.

But during the inquiry consumer advocate Michael Fraser, who said he alerted 7-Eleven to the problem in 2012, questioned how senior management did not know of the problem.

"How does an Indian in Melbourne, an Indian franchisee and a Pakistani franchisee in Sydney, and a Chinese franchisee in Brisbane, how do they all know the same scam?" Mr Fraser asked during the inquiry.

"How is it possible that me with no budget can stumble on such a big wage scandal buying a loaf of bread yet head office with all their oversight find no systemic problem?"

7-Eleven said it should know next month how many of its 620 franchisees underpaid their staff.

Calls for workers to be given visa amnesty

Tens of thousands of workers have potentially been affected. Most are on visas restricting their working time to 20 hours per week.

Many have been forced to work 40 hours, but paid for only 20.

Complaints were met with threats that they would be reported for breaching their visas.

It has led to calls from lawyers that anyone who has breached their visa conditions while working for 7-Eleven be given an amnesty.

The Greens member for the seat of Melbourne, Adam Bandt, said it was crucial the workers were able to give evidence free of any threat of deportation.

"Something is wrong in Australia where the head of 7-Eleven is a millionaire if not a billionaire and the people who are working in 7-Eleven stores are getting paid for only half of the hours they work and threatened with deportation back to their home country if they complain," he said.

"We've heard from many, many people now that they want to complain, to be paid the fair Australian minimum wage, but they're told that if they do it, they'll be reported to the Immigration Department and deported.

"Now that's not fair.

"These people are working long hours for low pay.

"They shouldn't also have to face [a] threat of deportation if they speak up and do the right thing."

7-Eleven has set up an independent panel to investigate the underpayments.