Bus was travelling 'erratically' before crash that sent 12 to hospital in south-west Sydney

One of the buses involved in a collision that left a dozen hospitalised in Sydney's south-west yesterday had been travelling "erratically" before the crash, it's been revealed.

Police said today 12 people had been rushed to hospital after two buses crashed at the intersection of Fifth Avenue and Seventh Avenue in Campsie about 4.20pm yesterday.

NSW Police say one bus collided into the rear of the other, then causing it to crash into three parked cars, but it is not known what caused the incident.

READ MORE: Driver stung over 'dangerously defective' seatbelt held together by tape

A witness rushed out of their house nearby and was able to open the door of one of the buses, freeing some of the passengers.

"The actions of that person not only assisted those passengers but also assisted emergency services," Detective Inspector Dean Johnstone said at a press briefing this morning.

Johnstone said one of the buses had been spotted travelling erratically along Seventh Avenue before the collision.

He said it had been swerving between lanes and putting on sudden bursts of speed, even having a minor collision with a tree.

One bus driver was trapped inside the vehicle along with a passenger, with firefighters having to pry open the smashed windscreen to rescue the pair.

The driver of one of the buses, a 63-year-old woman, sustained serious injuries, according to NSW Police.

She was treated by NSW Ambulance paramedics before being taken to St George Hospital in a critical condition.

The other bus driver, a 63-year-old man, sustained minor injuries and was taken to Concord Hospital in a stable condition.

NSW Ambulance also said one female passenger, in her 50s, was seriously injured, suffering spinal injuries.

Johnstone said 12 people were taken to hospital, and that no children were among the injured.

READ MORE: Man dies from snake bites after trying to remove snake from childcare centre

There were close to 20 ambulances on the scene with at least 25 passengers treated by paramedics. Thirteen of those did not need to go to hospital.

NSW Ambulance said there was no evidence of a medical episode but it was not being ruled out.

Johnstone likewise said the police investigation would include the possibilities of a medical episode as well as a potential mechanical fault.

He appealed for anybody with dashcam footage to contact police.

FOLLOW US ON WHATSAPP HERE: Stay across all the latest in breaking news, celebrity and sport via our WhatsApp channel. No comments, no algorithm and nobody can see your private details.