Police said the two-vehicle crash occurred at the intersection of State Highway 1 and Looker Rd, near Rangitata around 3:30pm.
St John initially said one person had been airlifted to Christchurch Hospital in a serious condition.
In an update this evening police said two people had died at the scene, and confirmed a third person had sustained serious injuries.
A witness earlier said a smoking car engine had flown into the middle of the road after the crash. He described the moment another bystander walked to one of the cars and hung a jersey over the window.
He said another bystander was "holding on to a body" on the side of the road.
He described seeing a black Holden saloon, and a red double-cab ute, the latter so badly damaged he "couldn't even tell the make of it - that's how bad it was".
Waka Kotahi NZ Transport Agency said the highway was closed and emergency services were at the scene. The serious crash unit was attending.
The highway has now reopened.
- ODT Online/NZ Herald
