The former operators of three NSW sushi outlets have been fined more than $380,000 for ripping off their employees, including several "vulnerable" migrants on working visas.

The regulator took the previous owners of three Toyko Sushi bars on the NSW Central Coast and Newcastle to court after they found out 31 employees had been underpaid in 2016.

Eight of the underpaid workers were juniors and a number were visa holders. The underpayments related to weekday pay, casual pay and penalties.

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Fair Work Ombudsman Sandra Parker said the regulator was cracking down on underpayments in the fast-food industry.

“Young migrant workers are particularly vulnerable workers because often they don't understand what their rights are in Australia," Ms Parker told SBS News in a statement.

"They are reluctant to come forward because they may end up having breached their visa, or they are not sure whether they may have breached their visa, and they are generally reluctant to deal with the government."

"So what we find is they are in a very vulnerable position when it comes to employers who want to exploit them," she said.

SBS

Regulators discovered the underpayments when they audited more than 40 sushi outlets, across NSW, and South-East Queensland.

"We have a number of annual priorities that we set, the fast-food sector is one of the areas we really want to hone in and crackdown, she told SBS News.

"There are significant issues in this sector."

She said inspectors will continue to conduct targeted audits across the fast food, restaurant and café sector and hold employers accountable if they are not meeting their obligations.

The ombudsman said workers who have any concerns should contact her office for free advice and help.

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Judge Philip Dowdy described the case as “serious” and said there was “no excuse” to rip off workers.

“The simple fact of the matter is that persons who engage in business activities which necessitate the employment of staff are under a strict obligation to pay their staff the just entitlements of the staff in accordance with [the] law,” Judge Dowdy said.

“Employees are entitled to respect and part of that respect is to pay them their full entitlements which must be recognised and known to the employer.”

Judge Dowdy said the penalties should deter employers who might "be inclined to contravene the Fair Work Act in a similar fashion”.

Mohammad Al-Khafaji, the CEO of Australia's peak multicultural body, FECCA, said migrant workers are at a greater risk of exploitation in the hospitality and retail industries because of a lack of understanding of their rights.

“We hope that the focus of the Ombudsman on these cases sends a message to employers who might be taking advantage of migrant workers that their actions are not only unethical, they have the potential to cause serious brand damage and attract heavy sanctions," he told SBS News.

It comes after the Fair Work Commission announced it would increase the minimum wage by three per cent to $19.49 an hour from the first of July.

Employers and employees seeking assistance can visit www.fairwork.gov.au or call the Fair Work Infoline on 13 13 94. An interpreter service is available on 13 14 50.