United Voice's Stefan Russell-Uren said outsourced labour contracts left little fat for workers wages, and alleged that hourly rates of between $7 and $15 were paid to staff often vulnerable because of their limited English or education.

"The union alleges that trolley collection companies repeatedly fail to pay the minimum wage, overtime, personal leave and annual leave," Mr Russell-Uren said.

"The head contractors know that the sub-contractors are non-compliant because they procure the state of non-compliance."

The Cleaning Services Award, which covers trolley collectors, sets a minimum hourly rate for full-time employees of $17.49. Junior employees – aged 20 and younger – are required to be paid a proportion of the full wage on a sliding scale.

Mr Russell-Uren said payments were often in cash, and and the collectors were typically from non-English speaking backgrounds, on visas that restricted work or on a government benefit.

A long-time Canberra trolley collector, who asked not to be named, said he was paid as little as $1000 a fortnight for more than a year from 2009, despite working 12-hour days and often seven days a week.

Complaints were slow to be acted on by the contractor 7 Master Pty Ltd, he said, and created a danger his manager would replace him with someone who would not speak up.

"I [was] scared that he’d sack me and I'd be scared that it would be all around Canberra," he said.

The collector, who eventually left the company, said he was paid "under the table" or through the bank, with no record of who made the deposit, and for most of his time had no written contract.

7 Master operations manager Ian Park said his company never directly employed trolley collectors and the man who paid the source was a subcontractor.

Mr Park said 7 Master obtained a declaration that minimum wages were paid on the weekly tax invoice from subcontractors, and rejected the union’s claim that company-to-company contracts made an illegal wage inevitable for collectors.

A Coles spokeswoman declined to comment on wages paid through subcontracting arrangements.

"We have strict policies and procedures in place to ensure the agreements we hold with our trolley collection suppliers in the ACT operate within the relevant regulations," she said.

"We cannot comment further on the specifics of these commercial agreements."

A spokesman for Woolworths confirmed the company used trolley collection companies in the ACT and did not directly employ trolley collectors.

“We work closely with the Fair Work Ombudsman to ensure we comply with all relevant laws,” he said.

The Fair Work Ombudsman has investigated the industry for many years, and their most prominent claim against one of the duopoly, Coles, was filed in 2012 and is listed for a 14-day Federal Court trial in October this year.

The Ombudsman has alleged Coles knew that under the contracting prices it paid to its head contractor it was not feasible to provide the required trolley services without undercutting minimum wage rates.

Court documents, relying on accessorial liability provisions in the Fair Work Act, allege that Coles was wilfully blind to underpayments made to collectors in Adelaide and did not take action to prevent them.

Coles filed defences in the proceedings last year.