Foreign workers have been exploited and underpaid at one of Australia's biggest chicken processors, according to an investigation by the Fair Work Ombudsman.

The ombudsman found Baiada, which produces Steggles and Lilydale brands, used a complex web of labour hire arrangements to outsource costs and risk at three processing plants in regional New South Wales.

Labour hire companies were engaged to provide Baiada with foreign workers on 417 visas, mainly from Taiwan.

The ombudsman said they were paid about half the minimum hourly wage and worked up to 18 hours a day without paid overtime.

"I don't believe that any worker in this country should be forced to work in those types of circumstances nor endure those types of practices by any employer," Deputy Fair Work Ombudsman Michael Campbell told Lateline.

The ombudsman's final report found much of the work was "off the books" with the foreign workers paid in cash.

The investigation was prompted by a Lateline expose in 2013, revealing the illegal underpayment of foreign workers at the company's Beresfield plant near Newcastle.

The ombudsman confirmed Lateline's findings and went further, uncovering the same problem at the Baiada plants in Hanwood and Tamworth.

It referred some of the labour hire companies connected to Baiada to corporate regulator ASIC for possible breaches of corporations law.

'What we see is like syndicated crime': Campbell

The ombudsman found, at the time, Baiada engaged six principal labour hire companies, which, in turn, were connected to numerous other entities.

During the ombudsman's inquiry, some of the companies provided fabricated records or vanished altogether.

"Unfortunately what this means for our investigation is that it's very hard to pin down an individual to hold accountable. In many ways, what we see is like syndicated crime," Mr Campbell said.

"You take out one player and another steps into their shoes."

The ombudsman found hundreds of thousands of dollars in payments from Baiada to the labour hire companies were unaccounted for.

Mr Campbell said it appeared some of the labour hire companies had skimmed the money and short changed workers.

The National Union of Workers (NUW) said there were cases of foreign workers being exploited at Baiada sites in Victoria, Western Australia and South Australia.

NUW industrial officer Alex Snowball said Baiada's complex labour arrangements were designed to protect the company.

"What this means is that Baiada is protected from any sort of prosecution or any sort of claim being made against them," he said.

"However, it allows these subcontractors to breach the law with almost no consequences."

Baiada refuses to let Fair Work into processing plants

During the investigation, Baiada refused to let Fair Work inspectors into the three processing plants in NSW.

"It's a reckless attitude from Baiada, they seem to think they are above the law," Mr Snowball said.

He said current laws were grossly inadequate to deal with Baiada.

Baiada chief executive Simon Camilleri declined to be interviewed but the company said it was "deeply concerned by the reports that came to light detailing workers' poor treatment at the hands of some contractors".

Baiada said it had already responded to the Fair Work Ombudsman's recommendations.

Immigration Minister Peter Dutton and Assistant Immigration Minister Michaelia Cash declined to be interviewed.