Executives at Woolworths were quizzed at length at a Senate inquiry on Thursday if they faced similar problems to Coles, which had its union agreement thrown out by the workplace tribunal. The two supermarket giants have similar workplace deals.

In 2016, the full bench of the Fair Work Commission, in a landmark decision quashed an agreement between Coles and the Shop, Distributive and Allied Employees Association (SDA) as it failed the "better off overall test".

Evidence in that Coles case showed 56 per cent of workers were paid less than the minimum rates of the award, the basic wages safety net.

The Senate inquiry into the substandard agreements between the SDA and big retail and fast food employers was in response to a 12 month Fairfax Media investigation.

That investigation revealed how the "shoppies" union deals left more than 250,000 workers paid less than the award and saved Australia's biggest employers more than $300 million a year.

Among the employers that benefited were Australia's largest including McDonald's, Coles, Woolworths, KFC and Domino's.

Woolworths government relations manager Alison Penfold told the Senate inquiry that there could be cases and circumstances where its workers were paid below award rates from its deal with the SDA.

When asked about a Fairfax Media investigation that more than 60 per cent of workers at a Melbourne Woolworths supermarket were paid below rates, Ms Penfold said they were unable to respond to that question.

Ms Penfold said they had not done an analysis comparing its agreement to the award, drawing incredulity from some senators.

Woolworths workers are still covered by an agreement that expired more than two years ago and Ms Penfold said they wanted to bargain for a new wage deal.

It comes after a key retail employer group conceded that the workplace tribunal was right to throw out the wage deal between Coles and the "shoppies" as it left so many workers paid less than the legal minimum rates of the award.

The comments from Nick Tindley, head of employment law at the Australian Retailers Association, came after intense questioning from Nick Xenophon Team senators.

Mr Tindley had earlier complained of how the workplace relations system was operating and that bargaining was "in a stalemate" due to how the Fair Work Act was being interpreted. But when pressed by Senator Xenophon, he said: "I don't think that there's a valid criticism of the system that prevents situations where a majority of employees are worse off."

KFC chief people officer Robert Phipps was also grilled on Thursday about his company's agreement with the SDA, which paid no weekend penalty rates and hourly rates only 9 per cent above the award.

Mr Phipps would not answer directly how it was possible for workers who were engaged predominantly on weekends or nights to be paid above legal minimum rates.

He said KFC was in a "positive place" with the arrangement.

Last Friday, Domino's driver Casey Salt took action in the Fair Work Commission to terminate an agreement covering casual drivers that includes a raft of pay and conditions markedly lower than the award.

Drivers receive no casual loadings, no penalty rates, a limited driving allowance, and work minimum shifts of just two hours.

The 2005 agreement is estimated to have left workers tens of millions of dollars a year worse off. The SDA in response to Ms Salt's application said it had already applied to terminate the Domino's agreement and made a rare admission it was "deficient".

The SDA has repeatedly denied evidence from the Coles case and elsewhere that a majority of workers from its agreements have been left paid less than the award.

The union is Labor's largest affiliate and has been a staunch opponent of social change on issues such as same sex marriage and abortion.

Do you know more? Contact us securely via Journotips

Follow Ben Schneiders on Facebook