Five people have died after the small charter boat they were aboard capsized, authorities in New Zealand said, in what may have been a collision with a whale.

Another six people aboard the boat were rescued.

Police said the 8.5-metre (28ft) boat overturned near the South Island town of Kaikoura.

Officers said they were continuing to investigate the cause of the accident.

Two rescue boats alongside a capsized boat in Kaikoura, New Zealand
Two rescue boats alongside a capsized boat in Kaikoura, New Zealand (TVNZ via AP)

Kaikoura police sergeant Matt Boyce described it as a devastating and unprecedented event.

“Our thoughts are with everyone involved, including the victims and their families, their local communities, and emergency services personnel,” Mr Boyce said.

He said police divers had recovered the bodies of all those who had died.

Mr Boyce said all six survivors were assessed to be in a stable condition at a local health centre, with one transferred to a hospital in the city of Christchurch as a precaution.

Kaikoura mayor Craig Mackle told the Associated Press that the water was 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 travelling through.

A rescue boat approaches a capsized boat in Kaikoura, New Zealand
A rescue boat approaches a capsized boat in Kaikoura (TVNZ via AP)

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

Mr Mackle said he had 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 had not heard about any previous such accidents.

Mr Mackle said the boat was a charter vessel typically used for fishing excursions.

News agency Stuff reported the passengers belonged to a bird enthusiasts’ group.

Police said they were still notifying the relatives of those who died and could not yet publicly name the victims.

Vanessa Chapman told Stuff she and a group of friends had watched the rescue efforts unfold from Goose Bay, near Kaikoura.

A helicopter and a rescue boat search for survivors off the coast of Kaikoura, New Zealand, Saturday
A helicopter and a rescue boat search for survivors off the coast of Kaikoura, New Zealand (AP)

She said that when she arrived at a lookout spot, she could see a person sitting atop an overturned boat waving their arms.

Ms Chapman said two rescue helicopters and a third local helicopter were circling before two divers jumped out.

She told Stuff that the person atop the boat was rescued and a second person appeared to have been pulled from the water.

Kaikoura is a popular whale-watching destination.

The seafloor drops away precipitously from the coast, making for deep waters close to the shore.

A number of businesses offer boat trips or helicopter rides so tourists can see whales, dolphins and other sea creatures up close.

Compliance agency Maritime New Zealand said it sent two investigators to the scene and would be conducting a thorough investigation once recovery operations had concluded.

Principal investigator Tracy Phillips said the agency “offers its heartfelt condolences to the family and loved ones of the people who have died”.