Four out of five businesses advertising Australian jobs on Chinese, Korean and Spanish language websites are offering illegal pay rates as low as $4.20 per hour, a new audit has found.

Two separate audits conducted this year and last year found 78 per cent of 200 job advertisements examined were offering pay rates below minimum award rates.

All of the Chinese and Korean advertisements were for jobs in NSW, mostly in Sydney. The Spanish advertisements for were for jobs around the country.

The audit conducted by Unions NSW identified widespread migrant exploitation and alleged wage theft, particularly in the hospitality industry.

The lowest rates discovered included $4.20 per hour for a nanny and $9 per hour for an office clerk which were advertised in Chinese. The minimum legal rate for both jobs is more than $18 per hour.

Cleaning jobs were advertised at pay rates as little as $10 per hour. Retail jobs offered as little as $12.09 per hour, an average underpayment of $7.36.

The overall average wage rate advertised was $14.03 per hour, representing an average underpayment in the hourly rate of about $5.28.

Every job advertised in Mandarin and Cantonese reviewed this year had offered salaries below legal award rates.

The report Lighting up the black market: Enforcing minimum wages found 200 job advertisements that collectively offered a total annual underpayment of $1.62 million.

The incidence of underpayment was worse this year compared to a similar audit taken last year.

In 2016, 74 per cent of all the jobs reviewed were advertised below legal rates compared to 83 per cent in this year's audit.

Across the two audits, the average advertised hourly rate in hospitality was $13.60, which is $5.79 below average minimum award rates.

Half the jobs advertised on Chinese and Korean language websites were in hospitality.

Unions NSW Secretary Mark Morey said the underpayment of overseas workers had become routine.

"It is now a systematic business model where people think they can pay what they see as community rates rather than the appropriate rate," he said.

"It is undermining businesses that are paying award wages and creating an uncompetitive environment for them. 

"There is a legal rate in Australia and people should be paying the appropriate rates."

Angela, a 32-year-old woman from Korea said a Korean restaurant owner in Chatswood paid $11 per hour in 2013, the year she arrived in Australia. She found the job through a Korean community website.

She spoke little English and said the other staff in the restaurant were all Korean.

"I knew I was being underpaid, but I couldn't complain because I didn't have any work experience and I would have lost my job," she said.

She eventually quit the job making sandwiches, waiting and working in the kitchen because of the poor pay and conditions.

Angela, who is now paid correct Award rates as a child carer, also had a cleaning job in 2013 that she found through a Korean contractor which paid her $15 per hour.  

Her first job in Australia was picking blueberries for which she was paid about $4 to $6 per hour. She said that she and three room mates were each charged $100 for accommodation in a house that was shared by ten people.

"We came to Australia because we heard good stories about this country, but we had bad experiences after we arrived," she said. 

The Unions NSW audit reported an extensive network of websites and social media pages targeting people from specific nationalities and language groups who were new to a particular city or town.

The pages provide information and advertisements for rental accommodation and travel advice.

Across the two audits, 87 advertisements were in Chinese, 72 in Korean and 41 in Spanish.

Mr Morey said migrant workers often knew they were being ripped off, but lacked the confidence, language skills and support to stand up for their rights. Their visa or residency status was often used to threatened them against complaining about underpayment and exploitation. 

"This is wage theft on a massive scale. And it's being perpetrated against people ill-equipped to fight back," Mr Morey said.

"Our research finds that some employers believe they can offer a 'Korean' 'Chinese' or 'Spanish' rate of pay. Your pay rate is not determined by passport or ethnicity. We are all entitled to Australian standards."

The Unions NSW survey included two random audits conducted in March 2016 and April 2017 of advertisements from the webpages of Hojunara (Korean), Sydney Today (Chinese) and a number of Facebook groups targeted at Spanish speakers from Latin America.

The advertisements were translated and businesses that did not include rates of pay in their advertisement were called to collect that information.