A key figure in Australia’s unions wants bosses who underpay workers jailed.

A string of scandals at the 7-Eleven, Caltex, Dominoes Pizza and Pizza Hut franchises have seen employees underpaid by tens of millions of dollars.

Senior union leader Tony Sheldon says if franchise owners took the money out of the till, it would be theft, and they could face jail time.

“They have stolen millions of dollars of payments to their own employees,” Mr Sheldon told the ABC.

“They see it as part of their own corporate strategy on how to minimise payment while maximising their own profit — and that's theft.”

Mr Sheldon, Secretary of the Transport Workers Union, said there should be a special court to “properly deal with this criminal activity”.

“It's seen in corporate life as just another part of business,” he said.

“If you don't get caught, you get away with it.

“If you get caught, well, you pay some people some money if they're game enough to go ask for it.”

Mr Sheldon refused the claim that his call was political payback for the Government’s Registered Organisation Act, which introduced harsh new penalties for unions.

He instead claimed that the Act was “clearly failing to hold employers to account”, and he wanted punishments for the actual act of theft.

“If an employee, or an organisation, takes up an underpayment claim — a wages theft claim — there is no jail term attached, or potential jail term attached to that particular case,” he said.

“[The business] should be held to account, just as an employee would be if they were to steal those sums of money.”

The union will push for the position to be taken on as official Labor Party policy.