A new study of the 457 visa system has found some workers are being exploited, but not as many as the government estimates.
The study found that while most visa-holders say they are happy with their working arrangements, some are being underpaid and say employers aren't meeting their obligations.
The Migration Council has conducted the most detailed study of the system in 20 years, surveying 3,800 visa holders and 16 hundred employers.
"Our survey findings do indicate that there's around about a two to four per cent issue with the program in terms of rorts of the program, said Carla Wilshire form the Migration Council.
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The study found that 18 per cent of visa holders reported discrimination, 5 per cent believed their employers were breaching agreements and 2 per cent reported being paid less than the legal minimum.
A quarter of respondents couldn't, or wouldn't, disclose their income.
The unions are concerned about the visas.
"They can be used to exploit workers who are held to ransom really under promise of citizenship or being sent back home by unscrupulous bosses," said ACTU President Ged Kearney. "And I have no doubt that our experience as trade unions is that this exploitation of workers does exist".
The Immigration Minister has estimated 10 per cent are exploited, or 10,000 of the 105,000 primary visa holders -- acknowledging that's guesswork.
But the survey also found 88 per cent were happy or very happy with their employers - 20 per cent higher than in the wider community. 71 per cent want to apply to stay permanently, and half generally do.
Based on the survey, the Migration Council is now drafting 10 recommendations for the federal government. Among them, it recommends to charge employers more for the privilege of bringing workers from overseas.
