A tradie has been taken to hospital in a critical condition after being struck in the face with metal at a construction site in Sydney's north west on Wednesday.

The horror incident on Shirley Street in Carlingford was initially reported as a gas bottle explosion, NSW Ambulance said.

Four crews, including the CareFlight helicopter, were dispatched to the scene just before 12.15pm.

"Initial reports suggested it was a gas explosion, however, after arriving, it was identified the man had been struck by metal," a spokesman said.

Paramedics treated the 44-year-old man for significant facial injuries and he was taken to Westmead Hospital in a critical condition.

"Incidents at worksite can be extremely complex and it shows just how quickly any situation can change," NSW Ambulance Duty Operation Manager Jennine Kiely said.

"Bystanders did a great job coming to the man's aid.

"Paramedics were able to work together with other emergency services under these tense conditions to do everything they can for the patient."

A SafeWork NSW spokesperson said the incident was being investigated.

"SafeWork's enquiries are ongoing, and further comment is not available at this time," they said.

Originally published as Tradie injured in horror worksite incident