The Construction, Forestry, Mining and Energy Union (CFMEU) says one building site in regional New South Wales is exploiting dozens of foreign workers, with some paid just four dollars an hour.

The union claims that 13 Chinese and 16 Filipino workers employed by Taiwanese company Chia Tung Development Corporation are working ten to eleven hours per day, seven days per week and earning $40 to $100 per day.

They all work on a project to install a pellet feed mill Manildra Group’s Ethanol Plant in Bomaderry.

In fact, the union officials say the workers have told them during site visits that they are just part of a national ring of exploitative employers, which could extend nationwide.

“We're getting tip-offs also from the public in a variety of different areas as well and we have a fairly substantial list of places where we know where this is going on and we're going to investigate every part of it,” CFMEU State Secretary Brian Parker told the ABC.

The CFMEU says it is working with the workers to make sure they get what they are owed, but warns the planned changes to visa rules will only lead to more exploitation.

Manildra Group said Chia Tung have issued a statement saying they were unaware of any situations where workers were being exploited, but pledged to take the matter seriously.

The claim adds to ongoing debate about foreign workers and 457 visa programs, which company executives say provide a vital source of labour to fill skills gaps.

Unions claim the 457 scheme is rife with abuse.

They have been further angered by new proposals from the Department of Immigration and Border Protection, which is looking to allow companies to bring in foreign workers for twelve month contracts, but without the workers having to meet previous language and skill requirements.

The proposed changes will also stop employers from having to conduct market testing to find local labour.

The unions and even professional associations such as Engineers Australia fear the move could see foreign workers inundate the skilled professional market.