Allegations of worker exploitation in Crust Pizza's ACT stores first came to light in September last year, when it was reported some staff were being paid cash in hand below-award rates.

United Voice, the union representing Crust's workers, fears the problems are widespread, and is calling for legal changes to make management legally responsible for failing to pay staff a proper wage.

Earlier this year, another Crust driver, Simon Cole, came forward.

He said he was paid $15 per hour cash in hand by Crust pizza in Kingston, leaving him out of pocket about $9 an hour pre-tax.

Mr Cole said he asked his boss about his entitlements and then claims he was sacked.

That prompted an unfair dismissal case and a compensation claim for the underpayments, said to be worth thousands of dollars.

He was repaid the money, and pressed ahead with his wrongful dismissal case.

Crust Pizza settled that case with him two weeks ago for an undisclosed sum.

In April, two former employees of Crust alleged underpayments were occurring in another two shops with different management teams.

Those claims meant half of Canberra's eight Crust pizza stores and four different ownerships were alleged to have underpaid staff.

Eleven of the 46 requests for assistance from Crust employees to the Fair Work Ombudsman between July 2011 and March came from the ACT.

United Voice ACT secretary Lyndal Ryan is concerned more staff are being underpaid, but that many, including those on 457 visas, feared the consequences of speaking out.

"I can't think of a bigger disincentive not to come forward," Ms Ryan said.

But she urged anyone with concerns about payment to come to the union, who would talk confidentially, inform them of their rights, and only press ahead with action if given a green light.

"We are very well aware of the things that people have to take into consideration when taking on their employer. We don't encourage them to be reckless in that area," she said.

She said Mr Cole's case, which came even after Crust faced public criticism for underpayments at its Gungahlin store, was surprising.

"Certainly, what it indicates is just how widespread these problems are. And we really do need some urgent action in this space," she said.

Ms Ryan called for legal reforms to ensure accountability for underpayments went to the top of companies.

"They've got to have a legal obligation, a moral one isn't enough," she said.

"There has to be absolutely clear legal obligations, if you are going to outsource work, if you are going to enter these supply chain arrangements, you have to enter that legal obligation."

Crust Pizza's Kingston franchisee conducted an audit of delivery drivers after Mr Cole's complaint.

The audit found errors of payment for two drivers, and Crust's head office said they had since been paid back.

The Retail Food Group owns the Crust Pizza franchise system.

A spokesman said in March that the Kingston franchisee had found most delivery drivers were being paid properly.