The peak union body in New South Wales says migrant workers are being paid far below national standards through job listings in foreign language publications and urgent action is needed to stop it.

Unions NSW found 80 per cent of a sample of online job advertisements in Korean, Chinese and Spanish publications around the country paid below award rates.

In a two-year investigation, translators employed by the organisation posed as jobseekers to gather data.

"It's just ridiculous, the amount of jobs that aren't paying the legal requirement in this country, particularly for people whose first language isn't English," Unions NSW secretary Mark Morey said.

"We found that four in five jobs were paying below award rates, which is 80 per cent."

The organisation wants to use its findings, published in a report today, to push for increased rights for unions to again enter workplaces and inspect pay logs.

"We're certainly going to be pushing to have right-of-entry powers increased for unions to inspect wages books," Mr Morey said.

"We'll be pushing for that at the state conference in NSW in the coming weeks.

"But also we'll be pushing further the Turnbull Government and a potentially incoming Labor government to have a position on ensuring that just because your first language isn't English, doesn't mean you should be able to be ripped off at work."

Migrant workers unaware of industrial rights

Angela, who migrated from South Korea and was employed as a translator for Unions NSW, said underpaying employers often try to justify the wages.

"Some people say, 'If you're working in Korea, they're paying about $5, then I'm paying you more than that. It's really bad, isn't it?' she said.

An online job listing targeting migrant workers offering below award rates. ( Supplied: Unions NSW )

When Angela first arrived in Australia she was picking blueberries and was paid between $4 and $6 an hour.

She said she did not realise she was being underpaid because she was not aware of her industrial rights, and because others around her were in a similar position.

"Me and my friend around me, the Korean people, we're very sure about this one — most companies will pay under [the award] rate," she said.

It was a similar story in Brisbane for Hong Kong-born Heilok, who said he was ripped off when he first arrived in Australia.

"I was a forklift driver in a warehouse, [where] I was paid $15 only per hour," he said.

Heilok called the Fair Work Ombudsman and discovered he should have been earning at least $20 an hour under the national award.

Part of the problem is employers figure there is only a small chance they will get caught. ( Supplied: Unions NSW )

Worker underpayment on the rise

Stephen Clibborn, a lecturer at the University of Sydney Business School and associate editor of the Journal of Industrial Relations, said underpayment of workers was on the rise.

"The last two decades, we've had a large increase in temporary migration and they're also concentrated in large numbers in certain areas, such as in the cities, and these are often vulnerable workers," Dr Clibborn said.

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He said part of the problem appeared to be that employers figured there was only a small chance they would get caught underpaying their staff.

"The Fair Work Ombudsman, who does an excellent job with the resources that they have, nonetheless is well and truly under-resourced," Dr Clibborn said.

"They have roughly 250 inspectors to cover every workplace in Australia.

''They are not, of course, able to respond to complaints, but the effective work they're doing is in strategic enforcement.''