One of Australia's leading ethical chicken brands is facing allegations of underpaying and intimidating its mainly migrant workforce.

Lilydale bills itself as the ethical chicken choice, offering poultry from free-range chickens and highlighting the grass on which its hens are said to roam in its marketing.

But ABC TV's Lateline program has revealed Lilydale's producer, Baiada, is under investigation over claims of unethical treatment of its majority migrant workforce.

Workers are complaining they are being bullied into accepting below award wages and that they are being forced to work excessive hours.

Recently sacked worker Anyuon Mabior says "people are being treated like they are not human... and people are not respected most of the time."

The Lilydale free-range brand belongs to the Baiada group, Australia's largest chicken processor, which also owns Steggles.

Hundreds of thousands of chickens go through Baiada's Adelaide factory every week.

In a first for Australia's current industrial laws, the Adelaide poultry processor was served with a surprise right to enter order allowing union officials to inspect working conditions.

The National Union of Workers has given Fair Work Australia a petition and signed statements from a third of the workforce, stating they had been "working under poor pay, harassment and fear."

Shortly after the order was served the ABC witnessed workers - who had just started for the day - running from the factory.

"Some cars have just pulled up driven by people we believe are supervisors and a group of Chinese women that probably started work an hour ago literally ran out of the factory, jumped in the back of these cars and drove off, it was quite remarkable," said Dave Garland, from the National Union of Workers.

Statements provided to the the ABC outline the claims of harassment and poor pay.

"I believe I have been underpaid since I have started employment at Adelaide Poultry," one wrote.

"I have seen workers bullied and shouted at at work", another wrote.

Two more statements spoke of punishment for speaking out about the conditions.

"I used to be a supervisor but I was demoted when I insisted workers be allowed a break after 16 hours," one statement said.

"I have been told if we join the union the company will be unhappy and we could lose our jobs," another employee wrote.

'Fresh workers means cheap workers'

The majority of the 300 workers are recent immigrants or people on work or student visas, and according to Mr Garland they are referred to by management as "fresh workers".

He says "fresh workers means cheap workers... cheap fresh chickens on the supermarket shelf and cheaper, fresher workers in the factory."

After four years at Baiada, Mr Mabior says he was sacked last month for complaining about racism by supervisors.

Mr Mabior came to Australia from the civil war in Sudan and says his treatment by Baiada reminded him of his time in Sudan.

"I feel I been treated the same way I was treated when I was in Africa... and I feel really horrible. I have to think how I am I going to handle myself so I don't get in trouble," he said.

He claims he and other Adelaide Poultry workers have experienced on-site punishments.

In his case for refusing to use a forklift in what he says was an unsafe manner, he says he was taken to where the chickens were killed as a punishment.

"I was sent home... when I come back I was taken to the termination site where they kill chicken using gas... and they took me there for a punishment. To make me feel like I have done something wrong... and then I was like 'is this how everyone is treated in Australia? Or is it 'cause all the workers here are migrants?'."

He says he was sent there for 30 minutes and felt really sick.

"I feel like I am contaminated because I inhaled some gasses and give me a lot of coughing and give me a lot of dizziness," he said.

Mr Mabior says many of his former colleagues cannot speak English and feel they cannot earn a living anywhere else.

He says they are afraid to raise problems at the factory, even something as small as not getting a payslip.

Some workers are employed at the factory through a labour hire company.

Those workers may be on visas limiting how many hours they can work and Mr Mabior says they are being forced to work extra hours after clocking off.

"They let people work like 40 hours per week and they are not getting money for those hours. When your 8 hours is finished the supervisor comes and says 'go and clock off'. When you clock off he says 'go back onto the company, go and work there'. If the person asks 'why do I? I have already clocked off. Why don't I get paid for these extra hours', they say 'you either do it or don't come tomorrow'," he said.

Workers have told the union the company name on their payslips has recently been changing every week and they say the rate of pay has been changing.

The union says payslips show the workers are often being paid below award wages and without some basic conditions like breaks and sick pay.

The NUW's Dave Garland says the shifting payslips "concern us because you know it looks like to us it could be companies shifting their liabilities."

South Australian Labor MP Leesa Vlahos has raised the worker's concerns in State Parliament.

"I feel sad and I have got goose bumps that it's in Australia," she said.

"I wouldn't say that I am really green in my approach to life but certainly human rights are just as important.

And this, latent beneath all of the blather about the enrichment of diversity, emigrees being the essence of our national being, us being a nation of migrants etc, is the reason why our government is keeping the immigrants coming to this country.