A huge multistorey crane has come crashing down on the Sydney Fish Market construction site, forcing the emergency evacuation of trades workers.

The tall structure’s arm is understood to have collapsed about midday Thursday, triggering chaos at the site on Bridge Road in Pyrmont.

A man in his 30s was taken to hospital after being treated for back injuries.

Footage shows the crane’s bent arm lying on a level of scaffolding after it smashed down from a height.

Witnesses described the loud crashing sound as the frightening accident unfolded and sent panic through workers.

The harbourside building site was locked down on Thursday afternoon as Safework NSW began an investigation into the cause of the accident.

The crane arm landed on scaffolding at the fish market construction site. Photo: Nine News

One nearby business owner told News Corp he heard a “massive thump” as the crane collapsed.

“It’s a significant collapse,” Earl White said, as he described emergency services rushing to the scene.

Trades workers have been evacuated from the site. Photo: Nine News

The Sydney Fish Market is an iconic local site that is being “reimagined” to create a market “for the future”.

The new purpose-built facility will have a distinctive wave-shaped and scale-patterned roof that pays homage to the fishing industry.

The NSW government hopes to turn the Bays Precinct into a major food and dining attraction on Sydney’s inner harbour.

It’s set to open in 2024 with visitation to double to over 6 million people annually.

Live Traffic NSW urged motorists to avoid the area.

“Traffic is slow both ways and there are a lot of pedestrians in the area. Slow down, take care and factor in extra time,” it said.

The Sydney Fish Market reimagined is due to open in 2024. Photo: NSW government