WELLINGTON - Five people died after a boat flipped over in waters off of New Zealand's South Island Saturday, with "indications" the vessel capsized after colliding with a whale.

The boat, which had 11 people on board, mostly from a bird-watching group, overturned in calm waters soon after 10am (6am Singapore time) at Goose Bay, about 15km south of Kaikoura, according to local officials.

A rescue operation that spanned nearly seven hours ended when a police dive squad recovered five bodies from inside the vessel.

Six people were rescued, having suffered minor injuries. Media reports said the skipper of the boat survived.

Earlier, images on social media showed survivors waving for help on the upturned hull of the 8.5m boat, which police confirmed belonged to a local charter business.

Kaikoura Mayor Craig Mackle said there were "indications" the boat collided with a whale. But police would not comment on that possibility, as their investigation continued.

"The information we have at the moment is it appears to be a collision. With what, we don't know at this stage," Kaikoura Police Sergeant Matt Boyce told a press conference. "This is an unprecedented event that has occurred, involving significant response from emergency services and members of the public."

Mr Mackle was perplexed at how the boat capsized in what he said were "flat, perfect" conditions, and confirmed it was a time of year when whales were prevalent off Kaikoura's coast.

"Humpback whales are coming through at the moment, and we have sperm whales that are resident," he said.

Kaikoura is a popular tourist attraction because of its abundant wildlife and boasts New Zealand's premier whale-watching experience.

"This is a tragic event that affects many lives, not least of all, family and loved ones," Mr Mackle said. "Our heartfelt sorrow and condolence go to them." AFP