Some of Australia's biggest mining and infrastructure projects are using workers who have gained skilled migrant visas or permanent residency by misleading the government about their employment status.

A Fairfax Media investigation can reveal that a multinational contractor, Murphy Pipe and Civil, repeatedly misled the Immigration Department to help Irish workers fraudulently obtain visas to work on the Queensland Curtis Liquefied Natural Gas project and West Australia's Sino Iron project.

The Sino Iron project mine in W.A.

Leaked files and well-placed sources have revealed evidence of widespread rorting of temporary skilled worker 457 visas and, to a lesser extent, working holiday visas.

But Fairfax Media has confirmed that the Immigration Department has failed to investigate the fraud, despite being contacted by whistleblowers over the past three years.