Highway Patrol officers attached to the Southern Traffic Group then stopped the car about 11.50am at the Hume Highway at Berrima.
There they discovered a woman in the boot of the car who'd suffered stab wounds.
The woman, 24, was treated at the scene before being taken to Liverpool Hospital. Her injuries are not life-threatening.
Two other women, aged 24 and 18, were arrested and taken to Southern Highlands Police Station.
The victim was stabbed in her knee, thigh and forearm, Detective Chief Inspector Brendan Bernie, the Crime Manager of The Hume Police District said today.
She was spotted by a truck driver, who saw her hand protruding from the tail-light cavity of the car, Bernie said he praised the driver for his "excellent work" alerting police.
"It's very fortunate we found this lady alive," he said.
"It could have ended up quite catastrophic for her and it's fortunate it ended in these circumstances.
"It's very rare the police have intercepted it in progress and it's very lucky nobody was killed."
He said they're investigating how the woman managed to break through the tail-light cavity and alert the public that she was in danger.
The victim and the two women in custody are all from the Greater Sydney area.
"Police are investigating how they came to be in the Southern Highlands," Bernie said, adding that early indications are that the victim had been in the boot for some time, allegedly since this morning.
Police are yet to interview the 24-year-old or 18-year-old woman arrested at the scene.
Bernie said it was too early to comment on possible charges, or motive.
An investigation into the incident is ongoing.
