Employers are continuing to exploit foreign students and they're getting better at covering their tracks, groups that represent migrant workers say.

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The Indian Workers' Association and the Migrant Workers' Association said employers were taking more care to ensure there was no evidence of deals that see people under-paid, over-worked, and charged thousands of dollars for jobs that provide work visas.

Anu Kaloti from the Migrant Workers Association said the organisation was getting more complaints than ever about abuse and exploitation.

"If I go back about a year or about six months ago, people were making contact maybe once a fortnight, once a month, but now it's definitely consistently two, three people who are contacting us every week," Mrs Kaloti said.

She said the increase might be due in part to increased awareness about the association and its success in getting compensation for some of the victims of exploitation.

But she warned that even employers who were under investigation by the authorities were continuing to sell jobs to former students for thousands of dollars.

"They have a very sophisticated way of receiving the money from the worker. It would be through several different accounts and each time that the worker pays them, the next instalment it comes through a different source, through a friend of a friend. They're making it very intricate and sophisticated so it can't be tracked or traced."

Mrs Kaloti said in some cases employers were selling jobs to former students for thousands of dollars, but then refusing to provide any employment at all.

The spokesperson for the Indian Workers Association, Mandeep Singh Bela, said the organisation and the First Union for which he was an organiser, had not seen an increase in complaints.

However, he said it was clear that employers were taking extra care to ensure there was no evidence of their exploitation of people on work or student visas.

"We have come across quite a few of them where the worker has said, 'oh yeah I wanted to record because otherwise I wouldn't have any evidence, but the employer asked me to put everything on the table, and asked me to switch off my cellphone, make sure I don't have anything else on me'."

The government needed to give foreign workers more protection from exploitation, Mr Bela said.

A particular problem was that some work visas were tied to a particular job and employer, he said, so people who complained generally lost their job and their right to work in New Zealand.

In situations where government agencies accepted cases for investigation, complainants should be given an open work visa, he said.

"So that migrant is not disadvantaged for coming forward. That is very important if we really want to address exploitation," Mr Bela said.

He said the government should also hold offshore agents accountable for misleading foreign students and workers, prosecute employers under the Crimes Act, and provide students and migrants with more information about their employment rights.

Labour Inspectorate figures showed more than half the investigations it completed each year involved migrants.

In the 2016/17 year it completed 406 cases involving migrants, and in 2015/16 it completed 433 cases.

The inspectorate's national manager, Stu Lumsden, said migrant workers had the same employment rights as anyone else working in New Zealand.

"Migrant exploitation is a priority for the Labour Inspectorate, and where we find premiums have been paid we will ensure they go back to the employee, and we will seek penalties against the employer," Mr Lumsden said.

There are currently 160 employers banned from sponsoring foreigners for work visas for periods of between six months and two years because they had breached minimum work standards, he said.

Anyone concerned about their employment situation, or the situation of someone they knew, should call the Labour Inspectorate on 0800 20 90 20, Mr Lumsden said.