Exploited 7-Eleven workers are still being forced to hand pay back to their bosses and at least one has been beaten for complaining, prompting former competition tsar Alan Fels to tell a Senate inquiry he has lost confidence in the company's ability to deal with the scandal.

Deloitte partner Siobhan Hennessy, a member of the Fels Wage Fairness Panel set up by the company to investigate the scandal, told senators in Canberra that a worker had been beaten for contacting the panel, saying that "the intimidation has gone to levels that are very worrying".

The hearing also heard testimony from former 7-Eleven chairman Russ Withers, current chairman Mike Smith and interim chief executive Bob Baily that up to 500 current workers were still being underpaid.

Back-pay claims related to the scandal were on track to reach up to $50 million in what Senator Sue Lines described as the largest back-pay claim in Australia's history.