Unionists are threatening to cut their support for the Labor party in Queensland over claims the state government has mismanaged its flagship rail project.

Hundreds of tradies protesting against the handling of the $5.4 billion Cross River Rail outside state parliament on Wednesday could be heard in its halls and in the chamber.

CFMEU state secretary Michael Ravbar has taken issue with the way contracts for the project have been awarded and says it isn't keeping state government promises to deliver on jobs for young people.

Queensland Premier Annastacia Palaszczuk at a press conference. (9News)

"It's too big of a project to allow the government to do the wrong thing by workers," he said.

"The construction industry is a big employer of people and it's a big voting base too, and if you don't look after your working class base, this Labor party's got some big issues."

Asked if the CFMEU would withdraw its support for Labor ahead of the October state election, Mr Ravbar said: "We've done that before, we did that with Anna Bligh."

"We didn't support the Labor government during that campaign of 2012."

Labor was obliterated by Queenslanders in that poll, reduced to just seven of the 89 parliamentary seats in a massive win for the Liberal National Party.

At the March 2012 election, the Liberal National Party (LNP) won a crushing 78 of the 89 parliamentary seats.

"If their policies and what they stand for are wrong, we don't care who they are," Mr Ravbar added.

He says responsibility for the project falls at Premier Annastacia Palaszczuk's feet.

"She's the leader," he said.

"The premier has to be the fixer, she can't keep blaming people around her."