The Immigration Department has confirmed it is investigating allegations Chinese workers on a West Australian iron ore project are being paid half the wage of their Australian counterparts.

The construction union says it has complained repeatedly about the treatment of the workers at CITIC Pacific's Sino Iron project at Cape Preston near Karratha.

The union's West Australian state secretary, Mick Buchan, says the workers are in a vulnerable position.

"The market rate for a non-tradesperson on that job, given the location, the remoteness, and the high-risk work they undertake would be around the $120,000 to $140,000 mark.

"We've heard these workers are paid half those rates.

"The feeling that I get is when the Australian workers get too close to the Chinese, or they make them accepted into Australia, bring them along to barbeques or take them fishing, either those workers get removed from the project or other influences come where the Chinese workers keep their distance from it."

The union's national secretary, Dave Noonan, says there needs to be a proper investigation into the matter to ensure workers are not being ripped off and that Australian labour standards are not being undermined.

"There are real and credible reports that have been put to the Immigration Department about this, and their lack of action on the matter is of great concern," he said.

The Department of Immigration says it will not comment further while its investigation is underway.

CITIC Pacific Mining says it has cooperated fully with the department during its recent audit of the project and will continue to do so.

The company says it requires that all contractors and sub-contractors on the project comply with all state and federal statutory requirements.