The survey of more than 1400 students by the University of Sydney's business school found that 80 per cent of international students working in restaurants across Sydney were being exploited, with up to 35 per cent of students being paid as little as $12 an hour.

Overall, 60 per cent of international students living in Sydney were paid below the federally mandated minimum wage of $17.29 per hour across all industries.

Retail was the worst offender, with 90 per cent of international workers being underpaid.

According to a Fairfax Media analysis of advertised rents in 2015, workers earning the minimum wage would be able to afford to rent in only 0.1 per cent of Sydney suburbs within 40 kilometres of the Sydney CBD. The suburbs are shown in blue on the map below.

To rent in The Rocks – the priciest location – a minimum wage earner would need to work nearly 22 hours a day, seven days a week.

But the minimum wage survey shows that the CBD and inner suburbs of Sydney continue to be hotspots for exploitation. Up to 43 per cent of underpaid workers work in central Sydney.

Researcher Stephen Clibborn described the results as "staggering".

"7-Eleven has been getting the headlines but what this shows is that underpayment is taking place in businesses right across Sydney," said Dr Clibborn, an associate lecturer in work and organisational studies at the business school.

Dr Clibborn said the lowest wage recorded in the survey was $0.

"It is common practice for people to work in restaurants and in shops for an unpaid training period for up to a month, at the end of which there may or may not be a job," he said.

In August last year, a joint Fairfax Media/Four Corners investigation found that 7-Eleven workers, many of whom were international students, were getting paid as little as $10 an hour. After travel expenses and tax, wages could slip as low as $5 an hour.

The chronic exploitation has damaged the image of Australia's second-largest export industry, education, now worth up to $17 billion. More than 400,000 international students are enrolled in Australian education institutions, often paying tens of thousands of dollars in fees while struggling to get by on below the minimum wage.

The practice has been well known to authorities since at least 2013, when a separate Fairfax Media investigation revealed more than 40 restaurants in Sydney were paying their staff wages as low as $8 an hour.

Last year, Wan, a 25-year-old Malaysian university graduate, said he was paid $13 an hour by the PappaRich Malaysian restaurant at Broadway and received no penalty rates or superannuation.

"They probably assume because we are students we have no idea of our rights," he told Fairfax Media. "This is a big international franchise, so it's no longer just small business."

In January, Fair Work launched an investigation into popular Malaysian restaurant Mamak for allegedly underpaying six employees by up to $87,000. The proceedings are continuing before the Federal Court.

The CBD restaurant, famous for its long queues and fast service, allegedly paid rates as low as $11 an hour to the casual staff over more than three years.

Dr Clibborn said that the Fair Work Ombudsman needed to be given more resources to investigate and prosecute infringements.

"We have a Fair Work office, they are doing a good job there but they have 2.1 million workplaces and 250 inspectors, so you can see the size of the task that they have got."

With Inga Ting and Amy McNeilage