A former Defence Force medic was among the first people on the scene of a serious crash that closed State Highway 1 for hours on Monday.
The main link between Picton and Christchurch was closed for about six hours near the Clarence River, north of Kaikōura, causing traffic to back up about three kilometres in either direction.
Police were called to the crash, between a southbound truck with trailers and a northbound ute, about 9.30am. The road was blocked by the trailers, and later by helicopters, and reopened about 3.30pm.
Marlborough highway patrol constable Andy Holmes said one person was seriously injured and the others had less serious injuries.
At least one person was trapped in the ute and was cut out, he said. The truck driver was uninjured.
Three rescue helicopters attended.
Karis Da Silva said she helped stabilise one of the crash victims at the scene of the crash.
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Da Silva applied a tourniquet to the driver of the ute.
"There was a lot of bleeding...I'm really glad we got the tourniquet on," she said.
Da Silva said a passenger got out of the vehicle and "seemed all right", but the former medic identified internal injuries, possible cracked ribs causing difficulty breathing.
"They both needed urgent care when I left the scene," Da Silva said. "It's been a while, I felt a bit rusty."
Da Silva, who was reluctant be named, said: "I really hope [the patients] are OK."
One person in a critical condition was flown to Wellington Hospital.
A Capital and Coast District Health Board spokeswoman said the person was undergoing treatment on Monday afternoon.
Another person in a moderate condition was flown to Christchurch hospital. A Christchurch hospital spokesman said they were in a "comfortable condition".
Christchurch Transport Operations Centre senior travel information adviser Steven Mann said contractors on the scene told him traffic backed up for about 3 kilometres on both sides at its worst.
He said traffic going through the Lewis Pass detour had been free flowing throughout the day.
A New Zealand Transport Agency spokesman said they had been able to clear traffic inside the closure before the road reopened.
He said they were able to manage queues by stationing people in Amberley and Blenheim to direct traffic onto the Lewis Pass.
Road Transport Association NZ chief executive Dennis Robertson said the crash would have been disruptive to trucks.
"It's just something that unfortunately the truck companies have to live with and manage … it will cost them money and time."
Robertson said drivers could legally only work for 14 hours a day so some would not be able to reach their destination before the limit, meaning some trucks would need a replacement driver. He said most trucks drove the highway during the day due to ferry schedules.