The collision between a car and a truck happened at the intersection of Moeraki Boulders Rd and the highway, near Hampden, about 5.10pm.
A Fire and Emergency New Zealand (Fenz) spokesman said staff removed a person from a vehicle.
Police said initial reports indicated there had been injuries.
A St John spokesman later confirmed a helicopter transported a critically injured patient to hospital.
Two other patients were treated at the scene, he said.
Ambulances from Oamaru and Palmerston responded.
The Fenz spokesman said crews from Hampden, Palmerston and Weston were called to the crash.
A police spokeswoman said SH1 was closed to northbound traffic at the site, but a southbound diversion for light vehicles only was opened through farmland by 6.45pm.
An Otago Daily Times reporter at the scene about 6pm said southbound traffic was backed up for about 2km.
The line-up of trucks unable to take the diversion later was "just incredible", stretching back about 1km.
As he passed the wreckage, through the diversion, he said the car had been "flattened".
A witness who passed the scene only minutes after the incident said a truck that had stopped near the wreckage had its cab door wrapped around the front of the truck as a result of the impact.
- Additional reporting Graham McKerracher
