A whistleblower at Gina Rinehart's Roy Hill iron ore project in Western Australia's Pilbara region claims workers on 457 visas are working excessive hours and being grossly underpaid.

The CFMEU says it has asked Employment Minister Eric Abetz to start an urgent investigation.

Accusations that workers at Roy Hill on 457 visas are working long hours for low pay Credit:Nick Cubbin

The whistleblower says up to 200 white-collar 457 visa workers, about half of whom are Korean nationals aged under 30, are clocking up more than 84 hours a week. Many are female.

They are employed by the contractor Samsung C&T and being paid about $16 an hour, the union says.