A large piece of scaffolding has fallen at a Sydney Metro construction site in Sydney's CBD, crushing the back of an Australia Post van and narrowly missing its driver.

Emergency services were called to the scene at 9:30am and found a van covered in debris from the collapse.

The 50-year-old driver of the vehicle was assessed for injury but did not require medical assistance.

A number of construction workers were interviewed by police at the scene, before workers were told to go home for the day.

Union officials from the Construction, Forestry and Maritime Employees Union (CFMEU) were also present outside the police exclusion zone.

The incident occurred in front of one of the Sydney Metro rail stations that was supposed to be opened last weekend, but was delayed due to safety concerns.

It's understood the platform was being moved from the building that was under construction above the southern end of the new Gadigal station.

Australia Post has confirmed two team members were involved but were not injured and are receiving support. 

There's traffic congestion on Pitt Street approaching Bathurst Street this afternoon, with only one northbound lane open as the clean-up continues.

Someone sitting in the halted traffic soon after the incident said the position of the person who relayed the reason for the disruption "was quite amazing". 

"I saw this gentleman walking towards me, he was stopping at every car and explaining what was going on," the man told ABC Radio Sydney.

"He came to my car and said 'oh hello, I'm [the chief operations officer] Howard Collins from Transport NSW.

"He said 'look, sorry to tell you this but you're going to be here for about 15 to 20 minutes because a crane has unfortunately come down on an Australia Post truck'."

"I said, 'Thank you very much Howard, this is service'."

Police investigations into the cause of the incident have commenced.

The northbound road is currently closed between Bathurst and Liverpool Streets, causing heavy traffic conditions.

Commuters travelling through the city have been advised to consider alternative routes.