The incident happened around 8.40 am when the 49-year-old supervisor, Cheung, stopped the crusher and used his right hand to clear polystyrene boxes that had been blocking the machine.

A worker Wong, who saw the incident, said a female worker "switched on the machine without noticing that Cheung's arm was still inside," causing his arm to be pulled into the crusher.

Another man was cleaning the machine from outside before seeing Cheung being dragged out by other colleagues, then immediately called the police.

Cheung did not regain consciousness until the ambulance arrived and began screaming in pain, Wong added, describing the arm as "shattered and broken."

Firefighters also arrived at the scene and rescued the semi-conscious man. He was then rushed to Princess Margaret Hospital.

Cheung was hospitalized after surgery in the orthopedic ward, with his wife in shock, saying "his arm was broken and he couldn't get it back " and needing to be comforted by friends.

It is understood Cheung, who has been working at the station since September, is responsible for management and operation of the crusher.

He has two sons in their 20s living in Hong Kong and the mainland.

The Environmental Protection Department said it had sent personnel to the scene and the hospital to understand Cheung's condition and the incident, as well as provide assistance to his family.

"It is noted that the West Kowloon Transfer Station contractor and subcontractor will give an ex-gratia relief payment to the injured worker," it said.

The EPD said it attached great importance to this accident and required the contractor and subcontractor to fully cooperate with the Labor Department's investigation, and requested a report on the incident from the contractor.

"In addition to regular inspections on the subcontractor's operation and occupational safety, the contractor also provides training on machinery operation and safety for subcontractors," it added.