A Melbourne woman has launched Federal Court action against fast-food company Grill'd, alleging she lost her job after complaining about being underpaid.

Kahlani Pyrah, 20, said she received a flat rate that was less than the award wage, and did not receive the shift loadings she was entitled to, while working at the Camberwell outlet.

"A few months ago, one of our friends was unfairly sacked, and people came to me because they knew I was a member of the union," she said.

"So I started looking into it ... and looked at our agreement to see what should or shouldn't be happening and I found out we'd been paid below the minimum wage for a really long time."

Ms Pyrah said the workers at Grill'd were employed under a WorkChoices agreement from 2007.

"I went to my workmates and told them what I'd found out ... we went to the bosses and told them what we'd found out asked them if they could change things," she said.

"It seemed like they listened to us, and they said 'we'll give you the right payments, we'll compensate people paid below the minimum wage, as long as you don't terminate your WorkChoices agreement'."

But Ms Pyrah said that there was no change in their wages, so she applied to the Fair Work Commission to have the agreement terminated.

Victorian secretary of United Voice Jess Walsh said Ms Pyrah was sacked 11 days later.

"Unfortunately underpayments in the hospitality sector, whether it is larger franchises like this or small, independent operations, underpayments are rife, and that's because, we think ... the workforce is predominantly young and casual and they get taken advantage of," she said.

Mr Pyrah said she wanted her job back.

"It's pretty clear that I was fired for standing up for my rights," she said.

Grill'd 'vigorously denies' underpayment claims

Grill'd said its workplace agreements were legally valid and ensured lawful rates of pay.

"These claims are vigorously denied by management and will be strongly defended," the company said in a statement.

The company said the Camberwell franchise was "a small family-owned" business.

"The owners run a values-based business that prides itself on providing a safe and enjoyable workplace for its employees," it said.

"The franchise partners at this restaurant work hard to make staff and community responsibilities their highest priority."

Ms Walsh said the 2007 agreement was "unfair" but not illegal.

"It's not an illegal agreement until it's terminated, this was an agreement that was made in 2007, just as the WorkChoices era was coming to a close, unfortunately it's been in place for many years since then and it's well under the award," she said.

A petition calling for Ms Pyrah's re-instatement has received almost 9,500 signatures since it was launched on Sunday.