Emergency personnel, including paramedics, police and fire crews, were called to the crash scene just before 1.30pm on Wednesday following reports of a truck and car accident on the Great Western Highway.

Multiple crews, along with a rescue helicopter with a critical care doctor and a critical care paramedic on board, were dispatched to the scene.

A gas leak from the damaged tanker posed as a hazard on scene and paramedics worked with Fire and Rescue NSW to ensure all safety precautions were taken.

Upon arrival, paramedics assessed two patients who, unfortunately, could not be resuscitated.

They then treated a man in his 70s for a suspected head injury, who was airlifted to Liverpool Hospital in a serious condition.

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Paramedics also treated a woman in her 20s for a hand injury and abrasions. She was transported by road to Lithgow Hospital in a stable condition.

Inspector Nigel Hardwick from NSW Ambulance said it was a terrible scene.

"It was an absolutely heart-wrenching sight. From the moment paramedic crews arrived on scene, it was clear how devastating the collision was," he said.

"Paramedics checked for signs of life on the critically injured patients, who unfortunately were deceased.

"It was a team effort between paramedics and other emergency services in difficult circumstances.

"Today our thoughts are with the families and friends who have lost loved ones."

The Great Western Highway remained closed in both directions late on Wednesday afternoon.

Police say an investigation into the crash is underway and anyone who witnessed the crash or has dashcam footage of the incident is urged to call Lithgow Police or Crime Stoppers on 1800 333 000.