After working at the local supermarket for five years, Lauren Morrison, 22, discovered this week that she had been underpaid by up to $5800 each year.
"It was a bit of a shock. It's a high amount of money," she said. "It's a bit devastating."
As a supervisor who works three days a week, her duties included managing staff and closing the IGA Glengala store.
She is not sure whether she will be able to reclaim the money she is owed in unpaid penalty rates but is relieved at the prospect of being paid the correct wage rate in the future.
"I don't think I'd get all that back," she said. "At least if I start getting paid the right amount, that would be good."
Ms Morrison works a second job in after-school care to help make ends meet but would not have needed a second job if she was paid her correct penalty rates.
"I'm studying at uni so I needed to get a second job. I won't need it if I'm getting the right rate," she said.
Ms Morrison is among 17 workers at two IGA supermarkets who have been paid as much as $8000 per year below their minimum award entitlements under a 2005 non-union agreement.
The Delahey Superfresh Pty Ltd Certified Agreement 2005 agreement has been used in two Victorian IGA stores, in Glengala and Delahey, covering about 100 people the union suspects have all been underpaid.
Their union, the Retail and Fast Food Workers Union, lodged an application to the Fair Work Commission on Tuesday to terminate the outdated wage agreement.
The contract has meant employees were not paid any penalty rates for weekend and night work in exchange for a base salary that was about 10 per cent above the minimum wage in 2005.
"The wage rate is now three or four dollars below the award rate," Josh Cullinan, secretary of the Retail and Fast Food Workers Union said.
"That kind of arrangement where penalty rates are cut or limited, we have come across that at every IGA we have looked at.
"We have got members at a range of IGAs and all those arrangements do similar things. They either cut Sunday rates altogether or in half and usually cut weeknight and Saturday rates as well."
Fairfax Media contacted IGA for comment but the call was not returned.
Mr Cullinan said the union planned to methodically terminate one IGA agreement after the other and up to 100 in total.
"It's where our members want it to be done. We are looking at three and will end up having to do all of them," he said.
Mr Cullinan said the employees should be paid penalty rates according to the relevant award after the so-called "zombie agreements" were terminated.
He said the agreements were required to be terminated upon application unless the employer could demonstrate it was against the public interest to terminate the agreements.
"We are finding more and more wherever a termination application is lodged, the agreement is terminated. So we expect this to be a quick process," Mr Cullinan said.
School student Tahlia Lemon, 16, who has worked on the IGA registers and grocery in Glengala for more than a year, felt "pretty frustrated and annoyed" when she discovered she was losing $1400 per year and "pretty relieved" the old agreement would likely be terminated.
High school student Khadija Acone, 17, said she worked weekends and early morning hours and found out she had been underpaid about $2700 per year.
Union delegate at the Glengala IGA store, Jimmy Siljanovski, 30, from Point Cook, contacted the union when he and other employees became frustrated at not receiving penalty rates. He had lost about $5000 a year in unpaid penalty rates across three years. He works 4pm to 9pm four days a week, including Saturdays.