Investigators say the driver lost control on an entrance ramp near Parramatta and went across four lanes of traffic.
The big rig came to rest on the shoulder then caught fire. Diesel and oil were also spilled during the wreck and they caught fire as well.
Traffic came to a standstill as the flames spread.
"It was getting bigger as we were approaching, we were concerned. It was getting bigger until about ten minutes after they arrived, it was spreading across the median there," witness Josh Sharp said.
HazMat crews worked through the night to clean up the spilled fuel.
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