MADEC's Chief Executive Laurence Burt of Mildura has been called to defend the actions of his organisation following a report on the Seasonal Workers Program published in the Sydney Morning Herald newspaper on Sunday.

The report claims that workers in South Australia’s Riverland were told that their work opportunities would be reduced if they joined a union.

The newspaper report also claims that MADEC staff in South Australia had prepared letters of resignation from the Union for the workers to sign,

Mr Burt told the Sydney Morning Herald that a "misunderstanding" had led to the distribution of the union resignation forms and that the company had taken urgent steps to assure its migrant workers they could join a union and exercise workplace rights.

"We are proud of our record in exposing bad practices in the industry," said Mr Burt of the not-for-profit labour hire firm that has recruited 3000 overseas workers for farmers, including 570 via the Seasonal Workers Program (SWP) in the last financial year.

The newspaper story states that the exploitation of workers by labour-hire agencies was dramatically exposed last November, when Fairfax Media partnered with an undercover Malaysian reporter to expose a major Australia stone fruit grower, Cutri Fruit, knowingly using undocumented and exploited foreign workers.

The federal government's Seasonal Workers Program is meant to be win-win, benefiting both under-employed fruit pickers from South Pacific nations and Australian farmers, who struggle to find local seasonal labour to ensure fruit isn't left to rot on the vine.

Affidavits signed by several of the workers allege that labour hire firm MADEC, Australia's largest user of the Seasonal Workers Program, had pressured workers to quit the National Union of Workers.

If they did not, they would be denied future employment, and workers from Vanuatu would not be recruited via the Seasonal Workers Program.

The Riverland farm company Perfection Fresh declined to comment to the Sydney newspaper.

National Union Workers W secretary Tim Kennedy claims "systemic exploitation" throughout the agriculture sector is the result of the price wars waged by major supermarkets, which he says is forcing farmers to squeeze labour-hire firms, who in turn recruit the cheapest possible labour.