A construction worker who died after being crushed by a scoop bucket at a south-west Sydney demolition site yesterday had been waiting for his wife to give birth today or tomorrow, his union says.

The 34-year-old man from Merrylands West was working at a Housing NSW demolition site at Riverside Drive, Airds, where townhouse precincts were being knocked down.

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He was standing near a tip truck being loaded by a 20-tonne single-arm excavator just before 3pm when the bucket came off.

It struck the truck and fell on top of him, causing fatal head and chest injuries, Inspector George Psaroudis of Campbelltown Local Area Command said.

Emergency services arrived and found him in cardiac arrest, a NSW Ambulance spokeswoman told the Campbelltown-Macarthur Advertiser.

He could not be revived.

It was believed the bucket did not have a safety pin at the time of the accident, a CFMEU (Construction, Forestry, Mining and Energy Union) spokeswoman said.

The safety pin is used to prevent the bucket from detaching from the arm of the excavator.

The spokeswoman said it was understood the demolition was completed and the workers were levelling dirt at the site when the accident took place.

The Department of Finance and Services extended its condolences to the family and friends of the man.

A department spokesman said the site would be part of the area set to be redeveloped under the Airds-Bradbury Renewal Project.

The man's wife had arrived in Australia from Lebanon about two months ago, the spokeswoman said, adding that it was believed he had been trying to bring her to Australia over the past year.

"The union is trying ... to help the widow," the spokeswoman said.

It was not known if the scoop had been full of earth or other demolition materials, Inspector Psaroudis said.

The man's family have been informed of his death, he added.

Police and WorkCover were investigating and a report was being prepared for the coroner.

The Airds-Bradbury Renewal Project is a partnership between the NSW government, a private consortium and a social housing provider.

It has been described as the biggest public-private partnership involving social housing in NSW.

with Megan Gorrey, Campbelltown-Macarthur Advertiser