A 150-tonne crane on a 400-tonne pontoon in the Brisbane River has escaped its moorings and could hurtle down the flood-engorged river towards the Story Bridge and Howard Smith Wharves.
It is the smaller of the cranes being used to build the Kangaroo Point Green Bridge, connecting the Stamford Plaza to Kangaroo Point.
Brisbane Lord Mayor Adrian Schrinner warned people near the Howard Smith Wharves precinct to evacuate immediately, while traffic would be cut on the Story Bridge if the crane moves downstream.
It comes off the back of an average 750mm of rain over the past five days and a major flood that has inundated more than 15,000 homes and much of the inner-city area and west to Toowoomba.
Police Minister Mark Ryan said the pontoon had been tethered down and engineers and Maritime Safety Queensland staff were working to shore up the tethers.
But with swift-moving floodwaters the barge presents a significant risk to river infrastructure and waterfront properties downstream, should the tethers break, and the Minister said the engineers were grappling with some "integrity issues".
The 150-tonne crane is believed to belong to LCR Lindores Group based at Hendra.
Connect Brisbane, led by BESIX Watpac, won the contract to design and construct the Kangaroo Point Green Bridge in June last year. Construction of the pedestrian bridge was expected to be complete in 2023.