One of NSW's largest services companies has been penalised after threatening foreign workers with dismissal if they joined a union.

Ventia Services, previously known as Thiess Services, hired 10 overseas Workers from the Philippines in 2013 to work on sites in Western Australia and Victoria under a contract effectively preventing them from engaging in union activity.

The group then lost their jobs or were reassigned, prompting them to contact the Electrical Trades Union, which raised concerns with Thiess and the Fair Work Ombudsman.

While it appeared to be an error of “poor governance” rather than a deliberate attempt to deprive workers of their rights, the union ban was still “black and white” in their contract and caused anxiety to the workers, Fair Work Ombudsman Natalie James said.

“The right of workers in Australia to join, or not join, a union is a fundamental freedom enshrined in the statute books,” she said in a statement.

Ventia is half owned by construction giant Cimic, which is building the WestConnex motorways project in Sydney.

As part of the make-up agreement with Fair Work, Ventia has to make a $50,000 donation to the Philippines-Australia Community Services group to help promote workplace rights.