NSW Police said the man was found trapped under stones after scaffolding at a building site at Fort Street High School, Petersham, collapsed before 11.30am Friday.
Paramedics confirmed the man could not be resuscitated and he died at the scene.
He is yet to be formally identified.
Fire and Rescue NSW Adam Dewberry said it will be a “complex operation to recover the person”.
“It’s likely we’re going to have to use a crane to start lifting off some of the debris,” Dewberry said.
Dewberry added it has left co-workers at the site distraught and students were kept away from the scene.
“The students have been moved away from the area and looked after,” he said.
“One of our firefighters took the time to talk to a co-worker to make sure he was alright.”
A crime scene has been established and Safe Work NSW has been notified.
