A woman and a child have suffered burns following a serious crash that blocked a major highway connecting Sydney with major centres in central-western NSW.

The child, who is under 10, was transferred by air to hospital in a serious condition following the collision, just after dawn on Monday.

Emergency services responded to the crash between a car and a truck on the Great Western Highway at Katoomba in the Blue Mountains about 6.30am.

Fire and Rescue NSW superintendent Adam Dewberry said the vehicle was on fire when crews arrived.

“Passers-by pulled both of the occupants, a lady and a child, out of the vehicle,” he said.

“Firefighters commenced emergency medical treatment on the child that was suffering some pretty serious burns.”

Paramedics treated the woman in her 30s and the young girl for burns at the scene before transferring them both to hospital.

The woman also sustained injuries to her legs and chest.

She was taken by road to Westmead Hospital, while the child was airlifted to Westmead Children’s Hospital.

A woman bystander in her 20s was also treated for minor burns and taken to Blue Mountains Hospital.

The driver of the truck was taken to the same hospital for mandatory testing.

A crime scene was established as specialist officers investigate the circumstances that led up to the crash.

The Great Western Highway was closed in both directions between Explorers Road and Medlow Bath.

Drivers were urged to avoid the area and use Bells Line of Road instead.

-AAP