A resident of a local farm said he heard a bang and went out to see what had happened. He could only see the truck - no cars were visible from his farm's boundary.
The crash happened right on the Main South Rd bend, he said.
Bridget Shimmin was on a southbound Intercity bus that arrived on the scene not long after the crash.
She said a car that was involved in the crash had ended up at least 100m down the road.
"[It] sounds like the truck clipped the car and spun out of control as a result. I think a couple of other cars closely missed the truck and had to swerve out the way."
The bus had waited at the scene while several paramedics who happened to be on board went to help, Shimmin said.
Once they reboarded the bus was redirected onto a gravel road that looped back to SH1 just before Rakaia.
St John was called at 8.45am, and five ambulances and a helicopter were sent to crash, a spokeswoman said.
Three fire appliances from Rakaia and Dunsandel were in attendance. Police were putting cordons in place, he said.
Fenz was called at 8.46am, Norris said.
SH1 BANKSIDE, CANTERBURY - CRASH - FINAL UPDATE 5:15PM
Vehicle recovery was successful and the road is now OPEN. Thanks...
Posted by Waka Kotahi NZ Transport Agency - South Island on Saturday, 9 November 2019
- More to come.