Convenience store chain 7-Eleven will get away with underpaying thousands of workers if staff are not offered visa protection for speaking out, a leading law firm has said.

The ABC’s Four Corners program last month revealed widespread exploitation of 7-Eleven staff, including underpaying them and forcing them to work longer hours than they are permitted under visa conditions.

The majority of the chain’s workers are foreign students on restricted visas and only allowed to work 20 hours per week, but the investigation found many were being forced to work up to 40 hours, putting them at risk of deportation.

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That investigation revealed more than two thirds of 7-Eleven’s 225 Australian stores had payroll compliance issues, with claims many franchisees would face financial ruin if staff were paid correctly.

An independent panel led by former Australian Competition and Consumer Commission (ACCC) boss Allan Fels has been set up by the company to help staff recover their wages.

But according to Giri Sivaraman from law firm Maurice Blackburn, many are not coming forward, with overseas students concerned they would lose their visas if they admitted to working too many hours.

He called on Prime Minister Malcolm Turnbull and the federal government to grant amnesty to staff who speak up.

“Without the amnesty, workers are very scared to come forward. They’re very scared about being deported,” Mr Sivaraman said.

“They’re already vulnerable and they’ve already been exploited, they already have a lack of trust in their employer and sometimes in the legal system as a whole.

“We’re talking about potentially thousands of workers who have been underpaid; this is one of the greatest employment law scandals in this country’s history.”

7-Eleven’s management said the rip-offs were not widespread and blamed the exploitation on its franchisees, but Four Corners reported head office was compliant.

Earlier this month, the Opposition called on executives to front a Senate inquiry into visa fraud and also pushed for students to be given amnesty if they come forward.

Maurice Blackburn said it would provide free legal advice to any workers trying to recover wages and other entitlements from the convenience store chain.

It has previously won cases against 7-Eleven stores over underpayment of employees.

Mr Sivaraman said workers would need to provide information such as rosters, timesheets and bank statements to prove what hours they worked as the company had “poor record keeping”.

“If they don’t come forward, it’s less money that 7-Eleven has to pay its employees who are owed it,” Mr Sivaraman said.