One person has died and five more have been seriously injured after a truck veered across three lanes of traffic and hit pedestrians waiting at a bus stop in Sydney.

Key points: The truck driver and a female pedestrian were taken to hospital in a critical condition

The truck driver and a female pedestrian were taken to hospital in a critical condition The B-double crashed into a building, which partially collapsed

The B-double crashed into a building, which partially collapsed It is understood the person who died was crushed by part of the building

The driver of the truck is fighting for his life in hospital and was trapped for 45 minutes after the crash, which happened on Botany Road in Alexandria about 7.45am.

A woman in her 20s was also taken to hospital in a critical condition and has undergone emergency surgery.

Ambulance officers described the scene as "absolutely horrific", saying pedestrians were strewn around a 50-metre area.

NSW Police Superintendent Andrew Holland said the truck had clipped a gutter before swerving into a power pole and into a group of people waiting for the bus.

"All pedestrians were on one side of the road and they've all been conveyed to local hospitals, and they are very, very serious injuries for the majority of them," he said.

He said the dead person was "knocked into the building by the sheer force of the truck".

"If you look at the front of the vehicle now it's still got traffic lights attached to the front of the truck," he said.

A pedestrian is taken away by paramedics near Green Square. ( AAP: Mick Tsikas )

NSW Ambulance Steven Vaughan said multiple people were found lying on the footpath and two other people were still sitting in their vehicles.

"The patients were spread over a significant distance, about 50 metres, with multiple people requiring multiple paramedics to stabilise their condition," Mr Vaughan said.

Police said they suspected the driver had experienced a medical episode.

Mr Vaughan said the male driver that was trapped in the front cabin of the truck was "critically unwell".

"Part of the truck [was] compressing his legs, which required significant intervention from the medical team and collaboration with police rescue and paramedics to remove him from the vehicle," Mr Vaughan said.

The driver was taken to Royal Prince Alfred Hospital in a critical condition, while the female pedestrian who was in a critical condition was taken to St Vincent's Hospital.

Police say the pedestrians have sustained "very serious" injuries. ( AAP: Mick Tsikas )

Several power line were brought down in the crash, and fuel spilt on to the road.

Nearby businesses remain without power and several staff from a construction zone across the road were sent home after witnessing the traumatic event.