The accident happened around 5am in the northbound lanes of the M1 Princes Motorway just past the New Mt Pleasant Road overpass.

NSW Ambulance Inspector Norm Rees said it had been reported that the 27-year-old woman was having an argument with her partner, who was driving up Mt Ousley.

The car pulled over to the side of the road, the woman got out and then began walking across the motorway.

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It is understood she was walking back towards New Mt Pleasant Road.

She was hit in the second lane of the motorway by another car.

Insp Rees said two off-duty police officers heading to Sutherland to start their shift were first on the scene.

The police officer driving saw what she initially thought was debris on the road before quickly realising it was a body.

She slammed on the brakes and both officers began to render assistance to the woman while waiting for ambulance crews to arrive.

The officers also managed to stop traffic while ambulance officers worked to treat the woman, who suffered multiple injuries, including fractures to her legs, arms and pelvis

She was airlifted to St George Hospital in a serious condition.

The accident saw the closure of the northbound lanes for several hours while the police Crash Investigation Unit worked at the scene.

During the closure, cars were diverted via Memorial Drive and Bulli Pass.

Heavy vehicles parked along the side of the road for several hours. The road was reopened around 10am.