A group of former waiters at popular Melbourne restaurant Hellenic Republic, owned by celebrity chef George Calombaris, claim they've been underpaid and the business is refusing to pay them back.

It comes after Calombaris' MAdE Establishment group last year apologised to 162 employees for underpaying them, and gave them backpay totalling $2.6 million.

The ABC has spoken to five former employees who say they were paid at a lower rate than they were entitled to under the award, because they were performing tasks at a higher grade than they were classified.

Payslips and email correspondence have been provided to support the claims.

One of the former workers has been back paid, but questions whether what he received completely covers what he was owed.

The former staff believe there are others in the same position, but it is unclear how many.

Former waiter Orlaith Belfrage, 25, believes she is owed $2,000 from her time working at the Brunswick East restaurant as a casual from March 2013 to May 2014.

She said she carried out a range of front-of-house duties, including serving customers, waiting tables and working at the bar.

"It was really great, they took me on and said they'd train me up as a waitress," she said.

George Calombaris' company has previously apologised to 162 employees for underpayment. ( AAP: Network Ten )

"[There was] definitely loads of positive experiences. The training was great, we learnt loads about different wines, food, service."

However, she said it was only in February 2018 she realised she was paid at a lower pay rate than she was entitled to, after being alerted by a colleague.

For much of her employment, she was paid at a grade-one level, but she claims her duties were a much higher level and included supervising staff.

A reference Hellenic Republic provided for Ms Belfrage states: "On many occasions she has been able to step up to higher levels of responsibility including running of shifts, coordinating staff and overseeing of day-to-day operation."

Ms Belfrage has made several email contacts with MAdE Establishment since February this year, arguing she had been classified at the wrong level.

However, representatives from the company replied in the emails: "Our records indicate that you were appropriately classified at the time of your employment."

'It's what's wrong with the hospitality industry'

Ms Belfrage said she was disappointed given Calombaris last year committed to ensuring all his employees were paid correctly and said that his team members were key to his restaurant group's success.

Adding to Ms Belfrage's concern is the fact that some of her former colleagues — who she said did the same job as her — have been back paid at the correct rate.

Aidan Carter said he was underpaid and has received backpay. ( Supplied )

Aidan Carter, 24, who worked on a casual basis for some of his five years at Hellenic Republic, raised issues with his pay with MAdE Establishment last year.

The company eventually agreed it had been paying him at a lower pay rate than he was entitled to under the award.

They initially offered him $1,200, but after Mr Carter questioned the figure, upped it to $1,400.

Mr Carter is unsure if the figure was exactly what he was owed.

He said it seemed unfair that he had been back paid, while his former colleagues — who did exactly the same job — were not.

"It's disgusting really. It's what's wrong with the hospitality industry at the moment," he said.

'"Last year the company introduced new company values, using the acronym LIFE. F stands for family. Well, you wouldn't treat your family like this."

Ms Belfrage said she was optimistic she and her other former colleagues would now be back paid given Mr Carter had been.

"I'm hopeful that our whole group will be back paid since, if we were all doing the same job, then that's what we're entitled to," she said.

"But I'm slightly frustrated that they haven't responded to all of us in the same way."

Company says it's 'made good all payments'

MAdE Establishment CEO Troy McDonagh said he could not comment on the cases as the restaurant group had not concluded its investigation.

However, he pointed to the business's track record or repaying staff.

"To date [we] have made good all payments due to both current and former employees," Mr McDonagh said.

"We are working very closely and collaboratively with FWO [Fair Work Ombudsman] as we have since April 3, 2017.

"[We are] committed to ongoing compliance with the restaurant award structures as we have clearly demonstrated from the very beginning."

Other venues in the MAdE Establishment group include The Press Club, Gazi and the Jimmy Grants chain.

These latest concerns relate only to Hellenic Republic in Brunswick East.

The Fair Work Ombudsman is investigating the latest claims of underpayment.

The office already conducted a major investigation into Calombaris' businesses in 2015-16, over concerns about staff being classified on the wrong awards and not being paid overtime.

MAdE Establishment hired auditor KPMG to examine its business and discovered it had underpaid staff to the tune of $2.6 million.

In April 2017, Calombaris said he was committed to paying everyone their full entitlements.

"I want to be clear that getting it right means ensuring that every single one of our team members is paid what they are entitled to under the industry award, and that any outstanding money owed to staff is rectified as our highest priority," he said at the time.

Calombaris, 39, began building his restaurant empire in 2006, with the opening of The Press Club on Flinders Street, in Melbourne's CBD.

Specialising in modern Greek cuisine, he has published numerous cookbooks and collected a number of awards over his career.

He became one of Australia's best-known chefs when he took on the role of judge in Channel Ten's MasterChef series in 2009.