Western Australian Treasurer Mike Nahan has launched a stinging attack on unions, saying he is "disgusted" by what he has described as "racially tainted" protests.

Hundreds of unionists joined Labor politicians to rally outside State Parliament yesterday in protest against Australia's Free Trade Agreement (FTA) with China.

They claim the FTA will allow Chinese workers to replace Australian workers on major construction jobs.

However, the Treasurer criticised the protest, accusing the unions of being racially motivated in their opposition to agreements with China and Japan.

"It's misinformation about the potential from these free trade arrangements both with China and Japan, they are hugely beneficial to Western Australia both for jobs and trade," Mr Nahan said.

"We worked on getting free trade arrangement, particularly with Japan, on agriculture for decades, and we now have the best arrangements with the Japanese on agriculture of any nation on earth, and they're trying to erode it, often with racially tainted terms, and it disgusts me."

State Secretary of the Australian Manufacturing Workers Union, Steve McCartney, denied the protests had anything to do with race.

"If [the Treasurer] believes a free trade agreement with China is going to create jobs, when it clearly is about taking construction jobs away for a start from Australian workers, I'm bewildered," he said.

"To put into place something that guarantees China the ability to undermine wages and conditions in Australia is a disgrace, and it's nothing to do with race, it's got everything to do with getting Australian jobs and keeping Australian families employed."