Police say it’s remarkable no one was killed when a massive concrete slab collapsed at a demolition site in the Sydney CBD yesterday evening.

Three pedestrians received minor injuries when the collapse caused debris and scaffolding from the work site to blow out on to the footpath on Pitt Street just after 4pm.

More than 20 workers on the site, where the old Greater Union cinema stood, were lucky to escape unharmed, police said.

It is not yet known what caused the collapse but the Construction Forestry Mining and Energy Union says a worker was operating an excavator on the site when he felt the slab beneath him vibrate strangely.

“We had a worker operating a machine on a 20 metre by 20 metre slab, he felt some rumbles on the slab, he felt the slab was about to go,” CFMEU assistant state secretary Brian Parker said.

“The slab did eventually go and when it went he rode it down in the machine.

“He had enough time to brace himself which was lucky.”

Mr Parker called on state and federal governments to set up a taskforce to investigate worksite problems.

City Central local area commander, Superintendent Mick Fuller, said the timing and location of the accident meant that many more people could have been hurt.

“We’re very lucky that no one was killed,” Superintendent Fuller said.