A sea of unionists has descended on Queensland Parliament House threatening to withdraw support for the Palaszczuk government ahead of next year's election if perceived problems with the Cross River Rail are not fixed.

The roar of hundreds of building workers from the powerful CFMEU could be heard from inside the walls of Parliament and drew a number of MPs to the outdoor balcony to watch.

A sea of high-vis descended on Queensland Parliament House protesting Cross River Rail. Credit:Lydia Lynch

"I could be here all day telling you the problems about this particular project," CFMEU state secretary Michael Ravbar said the crowd.

Mr Ravbar criticised Cross River Rail chief executive Graeme Newton, who he said "couldn't build a cubby house".