Twenty-two employees were ripped off more than $83,000 over two years, the Federal Court has found, with individual underpayments ranging from $219 to $17,000.

Workplace investigations at Gloria Jean's Caulfield East uncovered illegal underpayments of the casual staff, mostly international students aged under 21 from non-English-speaking backgrounds.

The franchisees have been ordered to pay substantial fines of more than $110,500 after the Fair Work Ombudsman won a major lawsuit.

Owner-operators Tsinman Fu and Ping Ostrovskih were also found to be repeat offenders regarding workplace law violations.

The pair previously controlled another company that faced serious penalties for undercutting worker entitlements.

The Gloria Jean's outlet is the latest business to draw attention to the problem of grossly underpaid young workers in the city's hospitality sector, which Judge Riethmuller said was "notorious for non-compliance" with industrial law.

In the largely cash industry, where requirements on wages and working conditions are routinely ignored, it is believed there are thousands of waiters, kitchen hands, cooks and dish-cleaners being paid well below the legal minimum.

The court found the owner-operators of the Gloria Jean's outlet made a "deliberate" decision to slash staff wages below the legal rate to match the business's declining cash flow.

Judge Riethmuller said the pair's previous misconduct meant they were "well aware of their obligations and the seriousness of those obligations".

The court dismissed the company's claims that the workers did not express dissatisfaction with their entitlements  before the Fair Work Ombudsman's probe.

"It is common for employees, particularly in industries where pay rates are low and workforces are largely casual, to be wary of expressing any dissatisfaction for fear that their employment will not continue," Judge Riethmuller said.

A Gloria Jean's spokeswoman said the company provided training and support for franchise partners' operations, but franchise partners were responsible for ensuring their businesses complied with employment law and the relevant awards.

Fair Work Ombudsman Natalie James said the court case sent a stern warning to other employers who were undercutting workers' minimum wage entitlements.

"We treat underpayments of young and overseas workers particularly seriously because they can be vulnerable if they are not fully aware of their workplace rights or are reluctant to complain," she said.

"Successful litigations such as this also benefit employers who are complying with workplace laws because it helps them to compete on a level playing field."

See theage.com.au for extended version of story.