Emergency crews were called to a worksite in Kingston and found a concrete pourer had fallen over, hitting three workers.
Firefighters say they managed to pull out a man in his early 20s trapped under the boom, but he died at the scene.
The two other workers suffered minor injuries and are receiving hospital treatment.
The construction union’s Jason O’Mara says the dead man had only been in the job for four weeks.
“This again is a tragic wakeup call for the industry. It’s our fourth fatality in Canberra in the last eight months,” he said.
Worksafe ACT says early investigations suggest equipment failure may have caused the accident.