A bus driving instructor has been killed during a training exercise in a western Sydney car park after the vehicle careered through a fence.
Emergency services were called to Fairfield Showground on Thursday morning after a 45-year-old man was struck by the bus.
The man died at the scene after suffering critical head injuries, NSW Police said.
Bus company Transit Systems confirmed "with great sadness" the man was one of its instructors.
"We are in a state of shock and grief and our sympathies are with the family of the trainer, who was much respected by his colleagues," Transit Systems chief executive Clint Feuerherdt said in a statement on Thursday.
The bus smashed through a fence separating the showground and the car park, coming to a stop on an embankment.
The driver was taken to hospital for mandatory tests, police said.
Retired heavy vehicle driver Ted Smith was working on a truck nearby when he heard the crash.
"I just heard a smash and people started screaming," he told reporters.
Mr Smith said he saw two buses, with groups of about 10 people each, doing training exercises in the car park shortly before the incident.
"Oh my god," Mr Smith said upon hearing the man died.
"Bad, bad, bad."
Multiple police units, including crash investigators, spent several hours at the scene following the incident.
An onlooker was treated for shock, a NSW Ambulance spokesman told AAP.
Mr Fueuerherdt said his company was co-operating fully with investigators and was providing counselling to staff and family.
"We've just lost a member of our family and we are devastated by this incident," he said.
The bus is owned by the state government's Transport NSW but operated by private company Transit Systems, AAP understands.
Transport Minister Andrew Constance said employees were being offered counselling.
"My thoughts and sincere condolences are with the family, loved ones and colleagues of the Transit Systems employee," he said in a statement.
"This was a tragic accident which NSW Police are investigating."
The Office of Transport Safety Investigations and SafeWork NSW are also investigating.
