Federal Opposition Leader Bill Shorten has apologised to the Australian arm of American sandwich and salad restaurant Subway after he incorrectly accused the franchise of ripping off workers.

In his media conference yesterday Mr Shorten was talking about the potential impacts of the China free trade deal on Australian workers.

"We want to make sure that we don't see people coming here on visas being exploited and undercutting Australian jobs," he said.

"I think we've all been appalled and disgusted by the scenes at Subway where literally thousands of people on visas are being ripped off."

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Mr Shorten has since apologised to Subway Australia on Twitter, saying he meant to refer to 7-Eleven, which has been the subject of an expose by Four Corners and Fairfax.

"Is it lunchtime yet? Might grab a sub," he tweeted.

Mr Shorten's office has since issued a transcript noting 7-Eleven in brackets after the mention of Subway.

The joint ABC/Fairfax investigation uncovered extensive evidence of what was described as "systemic wage rorting" within 7-Eleven.

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 correct wages.

Mr Shorten released a statement defending his comments and calling for an official investigation into the claims.

"What's been reported to be happening at 7-Eleven stores is just appalling," he said.

"If workers are being ripped off like this the authorities should be urgently investigating.

"Why is the Government so quick to pile in on its political opponents but missing on action when workers are being exploited?

"If this brings more attention to the plight of these workers, then I welcome that."

Subway declined to comment.

Earlier this year a Four Corners investigation revealed that some food being sold in Subway was being picked, packed and processed by gangs of black market workers run by unscrupulous labour hire contractors.

Woolworths, Coles, Aldi, IGA, Costco, KFC and Red Rooster were also implicated.