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Police were called to reports of a truck rolling about 8.50am on the Main South Rd bend today and arrived at the scene soon after.
Two people were killed in the crash and seven other people have moderate to minor injuries, police said.
A resident at a farm nearby said he had heard a bang and went out to see what had happened.
He could only see the truck involved - no cars were visible from his farm's boundary.
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 said diversions are in place at Old South Rd and Main Rakaia Rd. They are expected to be in place for some time and drivers should expect delays.