Firefighters are concerned that a large fire in the basement of a building site at Sydney's Barangaroo could bring down a construction crane.
Firefighters have detected a slight movement in the crane mounted directly above the area where the fire started.
"They detected just a very slight movement but not enough to cause concern," said Superintendent Ian Krimmer from NSW Fire and Rescue.
"We have laser beams to tell us if there is any movement."
Six fire trucks have been sent to battle the blaze, which erupted at the harbourside site around 2pm, forcing the evacuation of workers.
Firefighters and police have blocked off parts of Hickson Road as they try to extinguish the blaze from above with an elevated water cannon.
The fire is believed to have taken hold in wooden formwork.
NSW Fire and Rescue Service say the collapse of the crane is not imminent but something they are preparing for.
"The concern we have is on top of this basement area is the base for the large construction crane," Krimmer told journalists at the scene.
"There's no immediate threat of this crane coming down but we have to contain this fire."
Superintendent Krimmer said water had been diverted to the scene to battle the fire and water from the harbour may also be used.
"We're pumping water from surrounding streets into this area," he said.
"Our strategy is to contain this fire to ensure stability of the crane."
He said firefighters were facing a tough job in putting out the basement fire were still trying to get to the seat of it.
"It's extremely hot. Any firefighter knows that basement fires are our worst nightmare, there is nil visibility ... it's all by touch and feel."
Images posted to social media show flames and smoke pouring out of the basement of an under-construction building on Hickson Road.
NSW Ambulance is urging people in the Sydney area to be aware of their health as heavy smoke continues to blanket the area, stressing the message to the elderly, very young and those with respiratory conditions.
However, at this stage there were no extra concerns about the toxicity of the smoke, according to a spokeswoman for Fire and Rescue NSW.
The smoke could be seen right across the city skyline.
Fire and Rescue NSW Superintendent Ian Krimmer said all workers had been evacuated from the site and there were no injuries reported.
A spokesman for construction giant Lend Lease said: “We evacuated the site and we are currently working with emergency services to put out the fire.
“Our focus right now is to ensure the safety of our workers and the surrounding community.
"We are working with the surrounding buildings to evacuate those at risk. There's a number of buildings that are potentially at risk."
A CFMEU site official had previously told journalists that construction company Lend Lease was unable to tell the union that all workers had been accounted for.
"What has happened down here as far as we're concerned is an absolute disgrace."
The area is being redeveloped as a residential and retail hub, with plans for a high-rollers' casino owned by billionaire James Packer.