The Electrical Trades Union said 457 workers were generally a last resort and should only be used when no suitable Australian workers were available, but it claimed Thiess placed illegal clauses in contracts allowing migrant workers to be sacked and deported if they joined a union.

The ETU says this highlights the risks of allowing large companies to self-regulate their use of 457 visa holders.

Thiess had employed at least 11 skilled migrants on these illegal contracts, with the linesmen carrying out electricity network maintenance in Western Australia and Victoria.

The union said the scandal raises concerns about whether similar practices may be occurring on other major infrastructure and construction projects, resulting in the exploitation of vulnerable workers.

“Allowing companies like Thiess to self-regulate is a recipe for disaster and illegality,” ETU assistant national secretary Dave Mier said.

“This is not an isolated incident.

“It is the latest is an increasing list of cases where unscrupulous employers are exploiting overseas workers.

“Both sides of politics have failed Australian and guest workers by allowing the system to be easily rorted.

“It’s time for an immediate review of all 457 visa employment contracts from companies given self-regulation powers to see just how rotten this system is."

Mier said transparency, regulation and governance needed to be restored to the sector.

“Without strong investigatory and compliance measures to properly determine what other abuses of the 457 process are being perpetrated, these injustices will be repeated,” he said.

“We cannot allow companies like Thiess to intimidate workers in Australia against exercising their employment rights, regardless of where the workers come from.

“We will be referring this case to the current Senate inquiry into temporary overseas workers visas.”

Victorian secretary Troy Gray said that the “brazen illegality” of the contracts raised questions about what other forms of coercion was being practiced.

“If these threats can be made in writing to migrant workers, by an iconic Australian construction conglomerate, what's happening everywhere else?” he said.

Thiess operates major infrastructure, services and mining contracts across Australia and the world, including energy maintenance contracts for Ergon and Energex in Queensland, Western Power in WA and SP Ausnet in Victoria.

It also provides energy and communications for resources operations off the northwest coast of WA, site environmental services for Central Queensland’s Collinsville Coal operations, management and maintenance of the Yallourn power station in Victoria’s Latrobe Valley and remediation for gasworks sites in Toowoomba, Queensland.

Thiess is a joint venture between Leighton Holdings, which will soon be known as CIMIC following its takeover by a Chinese concern, and new partner Apollo Global Management.