Detectives are probing how a teenage girl came to be unconscious on the side of a highway after she was found by two passing truck drivers early this morning.

The 13-year-old was discovered on Henry Lawson Drive near the Hume Highway at Lansdowne in Sydney’s west about 1am.

WATCH THE VIDEO ABOVE: Teen found unconscious on Sydney highway

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She was rushed to hospital and police are investigating how the young teenager was injured.

The two truck drivers who found the girl stopped to help before emergency services arrived at the scene.

An investigation into the circumstances surrounding the incident is ongoing.

Emergency services attending to the girl. Credit: 7NEWS