The cyclist was taken to St Vincent's Hospital with serious injuries after he was trapped beneath the truck's chassis following the crash on the corner of Foveaux and Bellevue streets at 8.45am.

A NSW Ambulance spokeswoman said paramedics were called to the intersection to treat a male cyclist, aged in his 30s, who was trapped under the truck.

Witnesses had rushed to help the man, who was screaming out in pain from under the vehicle.

A medical team, including a doctor and intensive care paramedics, treated the cyclist for 30 minutes as he lay under the truck.

Initial reports suggested he was not seriously injured, but a NSW Ambulance spokesman later said the cyclist had suffered multiple injuries, including a suspected fractured pelvis and lower limb fractures.

Another cyclist said he came across the scene shortly after the crash occurred.

"I was riding down the hill, and when I got down there they were blocking off Foveaux Street. The cyclist has gone under the garbage truck," he said.

Police are investigating the cause of the crash.