07/03/2017 9:25 AM AEDT | Updated 07/03/2017 6:43 PM AEDT

Two Dead After Horrific Collision On Hume Highway

A 49-year-old woman and a 7-year-old boy have died.

Two people are dead and a 7-year-old girl is in a critical condition following a horrific car crash on the Hume Highway south of Sydney.

A 49-year-old woman and a 7-year-old boy were killed after the cars collided before 8am, emergency services say.

A 7-year-old girl who was also in the car is in a critical condition at the Children's Hospital, Westmead.

Another 49-year-old woman who was trapped for some time after the crash was taken by NSW Ambulance paramedics to Liverpool Hospital.

Emergency services responded to reports of a head-on collision in the south-bound lanes of the Hume Highway at Pheasants Nest, south of Sydney just before 8am.

Authorities initially said three people were killed, but Police later issued a clarifying statement saying two people had died.

One of the vehicles had rolled onto its roof as a result of the collision, with motorists saying that smoke was seen coming from the wreckage of the vehicle.

Two rescue helicopters landed on the highway to treat several patients, according to the Sydney Morning Herald.

The circumstances surrounding the crash are not yet known, however the Transport Management Centre has warned south-bound motorists to expect significant delays.

Crash Investigation officers are expected to attend to the scene.

More to come.