Researchers say immigrant workers are becoming trapped in a cycle of exploitation from workplace to workplace, but not enough is known about the problem to solve it.

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It has led to academics and human rights groups joining forces to start what they believe to be the most in-depth research of its kind in New Zealand.

The two-year study, carried out by the University of Auckland and non-government organisations, will assess the spectrum of exploitation from workplace bullying to human trafficking.

A researcher at the university's New Zealand Asia Institute, Glenn Simmons, said immigrants get trapped because of debts they accrue while trying to reach the country.

He said problems often arise because many work visas tie a person to working for one specified employer.

"They are reluctant to complain or to report the problems to authorities because they fear their visa will be cancelled - then they'll be sent home and not [be] in a position to repay those debts."

Amit Sharma's work visa stated he could only work at a particular fast-food restaurant in Auckland, but he said he had to work many hours of unpaid overtime.

"If you are working on a contract then, yep, you have to work more than 40 hours although they will pay you for 40 hours," he said. "My visa was company-specified, so I was supposed to work for them only."

He said, when unions became involved, he was moved to a different restaurant but the same thing happened.

Mr Sharma said he was fired on the day his visa ran out and he was listed as an illegal immigrant. He said he was now on a student visa.

Mr Simmons said he would like to interview about 1000 people for the study, and that the results from the project could be used to inform government policies.