'We heard a loud bang': Three dead after horror truck crash

Three people are dead and two others injured after their car hit a truck and ploughed headlong into a fence in Sydney's southwest.

Authorities were called to Campbelltown Road near St Andrews Road in Bow Bowing just before 11.30am after the crash.

The 74-year-old man driving the car and two passengers, a man and a woman, were treated by paramedics, but died at the scene.

A 76-year-old woman and 45-year-old man, who were also in the car, were taken to Liverpool Hospital in a stable condition by paramedics.

“We just heard a loud band and a crash, like a really loud sound, I looked out the window and saw a lot of police, ambulance going doing the street,” a witness told 9NEWS.

The driver of the truck, a 59-year-old man, was taken to Campbelltown Hospital for mandatory testing.

“It's pretty beat up, so you can understand why they've all passed away and what's happened to them, it's not, not a very nice sight, that's for sure,” local Greg Hays told 9NEWS.

The road was closed for five hours and police are investigating.