The Barangaroo blaze in inner Sydney has sparked calls for an urgent city-wide audit of construction safety standards from the state's peak union body.

Unions NSW secretary Mark Lennon will write to Finance Minister Andrew Constance on Thursday following what he says is the the third major scare on a building site in less than 18 months.

The push comes after flames broke out at the Barangaroo construction site on Wednesday afternoon, forcing the evacuation of hundreds of workers and threatening the collapse of a multi-storey crane.

Two weeks earlier, a scaffolding rig collapsed at a site in Mascot’s O’Riordan Street, injuring three.

And in November 2012 a crane at the University of Technology Sydney construction site at Broadway caught fire and caused cables to give way, sending the boom crashing down onto the site.

The crane crew was credited with saving the lives of building workers and members of the public.  Mr Lennon said each incident shut down major roadways and could easily have ended in tragedy.

“We’re very fortunate that neither a worker nor a member of the public has been killed,” he told AAP.

“WorkCover needs to conduct an audit of all major construction sites in Sydney to ensure appropriate safety measures are in place.”

He said a statewide audit would be ideal but the situation in Sydney was most pressing.

“Construction is booming in Sydney,” Mr Lennon said.

“If you look at the major sites such as Darling Harbour, Barangaroo, what’s happening around Green Park – there’s got to be at least 20 significant construction sites within a five-kilometre radius from the city.”