Police, fire and ambulance officers were called to the accident scene, about 200 metres from Silva Road and the Hawkesbury Road overpass, shortly after 11 am.

Police say a westbound white Commodore driven by a 73-year-old Koorawatha (Central West) man with a 51-year-old female passenger crossed the median strip to the other side of the highway.

The Commodore collided with an eastbound Subaru Forester with a 60-year-old South Penrith male driver and 67-year-old South Penrith woman on board.

A third vehicle, an eastbound Mazda 323 hatch with a 52-year-old female driver from Minchinbury and a 69-year-old Tregear woman passenger, was also struck by the Commodore.

Police say it is unknown why the Commodore went onto the wrong side of the highway and there is no indication that speed was excessive.

A helicopter landed on the highway and transported the female passenger from the Commodore to Westmead Hospital where she was in a critical condition with head injuries.

The driver of the Commodore was taken by ambulance to Nepean Hospital for treatment of a fractured sternum.

The female passenger from the Forester was taken to Nepean Hospital for treatment of fractures to the spine, hip and foot while the driver was assessed for light injuries at the hospital.

The women in the Mazda hatch were treated at Nepean Hospital for soft tissue injuries.

Traffic chaos clogged the Blue Mountains’ main artery, the Great Western Highway, for more than six hours, delaying locals, tourists heading back to Sydney from the Winter Magic Festival weekend and even a local football game after a referee could not get to his game on time.

Police closed off two westbound lanes on the highway at Springwood and diverted eastbound traffic through to the Hawkesbury Road exit and via Springwood town centre.

Normal traffic routes were re-established just before 5pm, but traffic remained heavy eastbound for several hours.