NINE people, including three children, have been injured in a shocking crash in Sydney's southwest.

The accident between a passenger van and a commercial van occurred on Mamre Rd, near Abbotts Rd, in Kemps Creek just after 1.30pm today.

Police said nine people were inside the passenger van and two others were in the commercial van when the collision occurred. A NSW Ambulance spokesman said there were "three paediatric patients", two of who were seriously injured, and all three have been taken to The Children's Hospital at Westmead.

Four adults and two teenagers were also injured in the crash and they were all taken to Liverpool and Westmead hospitals for further assessment and treatment.

NSW Ambulance Superintendent Paul Edwards said this accident was a "timely reminder going into Christmas to be careful".

"The scene was very distressing with three children involved - two of them with serious injuries," Supt Edwards said.

"We responded with 14 ambulance vehicles and two helicopters. We also called in three NSW Ambulance chaplains to support our staff, families and bystanders.

"Our chaplains are continuing to debrief with staff and family."

Mamre Rd was closed in both directions as emergency crews and crash investigators deal with the incident.

Drivers were being diverted along Bakers Lane, Aldington Rd and Abbotts Rd.

In chaotic scenes, one of the vans was pushed off the road into an embankment where paramedics crowded around the door trying to help a trapped patient.

One woman was seen to be carried to an ambulance on a stretcher while a man hobbled away from the wreckage supported by a friend.