A helicopter flies overs an upturned boat with a survivor sitting on the hull off the coast of Kaikoura, New Zealand, on Saturday. AP

Police said the 8.5-metre (28-foot) boat overturned near the South Island town of Kaikōura. Police said they were continuing to investigate the cause of the accident.

"This has been a tragic event and the police are providing support to those involved at this very difficult time," Kaikōura Police Sergeant Matt Boyce told reporters at the scene, news website Stuff reported.

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Kaikōura Mayor Craig Mackle told the media that the water was dead calm at the time of the accident and the assumption was that a whale had surfaced from beneath the boat.

He said there were some sperm whales in the area and also some humpback whales traveling through.

He said locals had helped with the rescue efforts throughout the day but the mood in the town was "somber” because the water was so cold and they feared for the outcome of anybody who had fallen overboard.

Mackle said he’d thought in the past about the possibility of a boat and whale colliding, given the number of whales that frequent the region.

"It always plays on your mind that it could happen,” he said, adding that he hadn’t heard about any previous such accidents.

Mackle said the boat was a fishing charter vessel.

Associated Press