A construction worker has been killed after a steel beam fell and crushed him while he was working on a barge in Sydney's Darling Harbour.

The rigger is understood to have been helping a crane driver move a heavy steel beam when it hit another beam and one fell, crushing him, just before 4pm on Wednesday.

"The top of his torso was crushed. Unfortunately he was dead instantly," CFMEU Secretary Brian Parker told AAP.

"The first people who got there have given that indication."

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The man, who has a young family, was understood to have been working on the Barangaroo ferry hub project for about six months.

Parker said the accident could potentially have been prevented.

"Nobody should be in the vicinity of the area when heavy lifting is being carried out. He shouldn't have been there," he said.

Ten News reported that hundreds of people were on a busy public path nearby, featuring several restaurants, when the incident occurred.

Speaking to Fairfax, Parker said: "One header beam was attached to the crane, so it's got chains which lift it, and that has hit another header beam that wasn't secured. So it was sitting in the air on two pylons. That has then fallen over and crushed the worker.

"Obviously the workers are very traumatised. I've spoken to a number of workers and I know they're all receiving counselling as we speak."

NSW Police and Safework NSW are investigating the accident.

NSW Infrastructure minister Andrew Constance has offered his condolences to the man's family and colleagues.

"In this horrific time for the man's family, friends and colleagues I would like to pass on my deepest sympathies and thoughts," he said.

"I trust our NSW government agencies will co-operate fully with all investigations into this very sad incident."