Two people have died in a helicopter crash at a river mouth in a small town north of Christchurch on New Zealand’s South Island.

Locals described their shock and horror as they witnessed the chopper spiral and crash onto a rocky beach near Kekerengu River at Clarence, about 200 kilometres north of Christchurch, on Tuesday.

The local witnesses – who beat emergency respondents to the remote accident scene – pulled passengers from the wreckage of the fallen chopper and towed it from the sea with a tractor.

New Zealand Police said a pilot and four passengers were on board the aircraft, which crashed at 2.40pm (AEDT).

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In a grim update soon after, they confirmed two fatalities and three serious injuries.

The surviving three people were flown to Christchurch Hospital. According to local media, they were all children.

Local resident Ian Mehrtens told the NZ Herald he thought the chopper was landing, until it started spinning around and around. After it crashed, he was among those who rushed to the beach and pulled people from the wreckage.

“I didn’t really count [how many] because it was just the stress of the moment to get people out,” he told Newstalk ZB.

Mr Mehrtens’ wife, Clarence bed and breakfast owner Lyn Mehrtens, said she saw the helicopter spiralling to the ground out of her kitchen window.

“I just saw the helicopter going round and round and it went down … I think it must’ve gone in the water,” she told Stuff.

“My husband just took off. He said, ‘god I’ve got to go’.”

“It’s scary. You’re just so helpless. You can’t do anything.”

Mr Mehrtens told 1News the chopper spiralled suddenly from about 30 to 40 metres above the ground.

“I went over to see if I could help and it was a bit of a mess. Bits and pieces everywhere and people screaming inside the helicopter,” he said.

“One wee girl was crying and calling out for her mum and the others weren’t too healthy at all.”

The region is known for whale-spotting helicopter flights, based from Kaikoura, about 30 kilometres south.

Emergency helicopters came from Christchurch, Nelson and Wellington to help with the aftermath of the crash.

Police have not released further information on those involved in the crash or whether any businesses have been involved.

New Zealand’s Civil Aviation Authority is investigating the fatal crash.

-with AAP