The tanker was coming down a steep hill on Mona Vale Road and overturned as it entered a roundabout, witnesses told the Australian Broadcasting Corp.
"I think he hasn't made it through the roundabout, and he's tried to correct the truck and flipped over," a witness identified only as Michael said. "It caught fire as soon as it hit the ground ... . The whole truck was engulfed in probably two or three minutes."
"One car went straight into the flames. A van behind that went up onto the hill. There was a lady in a BMW who went onto the side of the road," said another witness. "There was one car that went straight into the flames, so I'm unsure but assuming they're the ones that passed away."
The driver of the tanker was able to extricate himself safely from his truck, the Australian broadcaster reported.
Firefighters evacuated nearby buildings out of fear the leaking fuel could pose a fire threat.
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