Inside the envelope were two bills from cleaning and catering company Spotless, totalling $12.54 and a letter saying she had been overpaid.

Ms Gray was surprised to hear from the company, considering she had retired from its Cleanevent division four months earlier. But perhaps Spotless – the shares of which have more than halved since December after it warned investors of a big profit plunge – received a greater shock when she issued a bill of her own, and for a much greater sum.

Ms Gray is part of scores of employees who were underpaid by Spotless after it failed to pay them public holiday or shift allowances working at Flemington Race Course or the Australian Grand Prix during a 10-year period.

In her case, she wasn't paid public holiday allowances for 11 shifts from 2009 to 2013, a bill totalling $605.

Spotless, which bought Cleanevent in 2010, says it has tried to pay back as many people as it can. However, an employee advocate says its efforts haven't been enough and workers are still owed thousands of dollars plus superannuation.

Ms Gray was paid the money she was owed in December, but it was the company's handling of the $12 overpayment bill that has disappointed her the most.

Mindful not to damage her credit rating, Ms Gray quickly paid the bill hoping that would be the end of it. She was wrong. Two days before Christmas a letter arrived from debt collectors.

Alarmed, she tried to call the debt collection agency to explain Spotless' failure to record her payment. But the debt collectors had already left for their Christmas break and she had to wait until they returned in January to show them a receipt, which proved she had deposited the $12.54 into Spotless' bank account about six weeks earlier.

"I thought it was pretty lousy to go to all this work for a lousy $12, considering all the years and all the hours I had worked an hour or half an hour over and never put it down," Ms Gray said.

"It would have cost them more to send out the $12 bill. It's silly."

Spotless paid Ms Gray the $605 in December. The company also paid her superannuation, although Ms Gray says she no longer has a superannuation account because she is retired and is unsure where that money went.

Others are still waiting for their money. Unofficial employee advocate Ken Holland realised Spotless hadn't paid workers public holiday and site allowance soon after giving evidence for the Royal Commission into Trade Union Governance and Corruption last year.

The royal commission uncovered an arrangement that saw vulnerable employees cheated by the Australian Workers Union, which Bill Shorten formerly led.

Cleanevent paid the union $75,000 during three years and gave the AWU lists of employees who, largely unknown to them, were added to the union's membership roll to artificially inflate the numbers.

Employment Minister Michaelia Cash said the arrangement was "disgraceful" and "dodgy".

Mr Holland said not only did the company do the "dodgy deal" the union, it failed to pay workers their proper allowances.

"They had a process to check if worker was overpaid, which is OK, they are entitled to those payments. But they didn't have a similar process to check if workers were underpaid," Mr Holland said.

He said many of his former colleagues, who were from overseas and work for Spotless as backpackers, were yet to be paid the money they were owed.

Kaylene Dyer, who worked for the company for 14 years, said it was up to employees to contact Spotless to claim missing payments.

She approached the company after, like Ms Gray, it billed her for an overpayment, totalling $55. It came more than six months after she was let go by Spotless.

"At first I laughed at it and ignored it. Then they sent me another one, then another one with a solicitor's name on it."

Like Ms Gray, Ms Dyer discovered she was entitled to penalties the company failed to pay her, totalling more than $1800, which the company eventually paid.

Robert Vertigan, another former employee who Spotless has repaid allowances, questioned why the company didn't have payroll software that would have automatically paid workers the correct remuneration.

"We are dealing with a major company that has now become a publicly listed company," Mr Vertigan said.

"This software would be installed as a matter of course for such a large organisation for the efficient workings of the payroll department and for auditing purposes. Was this computer software installed? If so, why are there so many discrepancies? Was this software overridden?"

A spokeswoman for Spotless said the underpayments "predates our management and oversight of Cleanevent".

"However, all of the employees who worked at the Grand Prix have had their wages audited in relation to these matters and we have written to all of these employees with the contact information that we have on file," the spokeswoman said.

"To date we have used, and will continue to use, our best endeavours. To claim entitlements, all that is required is current bank account details. There are sound mechanisms in place for employees to escalate payroll issues and we always deal with these issues promptly. Spotless is fully committed to paying people due entitlements."

The spokeswoman urged employees who were owed money to contact the company via the email GPAllowanceQueries@spotless.com.au