•Craig Mackle, the mayor of Kaikōura, told reporters he believed the boat had hit a surfacing whale.
• Mackle said conditions in the bay at the time were "perfect" and that officials assumed the whale had surfaced beneath the vessel, causing it to overturn.
Five people have died in New Zealand after a birdwatching boat capsized, possibly after colliding with a whale.
Eleven people, mostly from the birdwatching group, were onboard when the boat capsized on Saturday in Goose Bay near the town of Kaikōura.
Police declined to speculate on what had caused the accident, merely confirming the collision.
But Craig Mackle, the mayor of Kaikōura, told reporters he believed the boat had hit a surfacing whale.
Mackle said conditions in the bay at the time were "perfect" and that officials assumed the whale had surfaced beneath the vessel, causing it to overturn.
If the boat had hit debris - for example a log - it would have left a large hole in the 8.5-meter (28-foot) boat, he added, which wasn't apparent.
"This is a tragic event that affects many lives, not least of all families and loved ones," Mackie said at a news conference.
"I would like to thank everyone involved in the rescue and the recovery. Being able to bring everyone home is the best result in such a terrible circumstance."
Sergeant Matt Boyce declined to comment on the theory, but admitted that the incident was "unprecedented".
He said survivors of the incident, including the boat's captain, had all been taken to hospital and later discharged. One survivor sustained minor injuries.
The vessel's passengers were part of a bird-watching group that is believed to have assembled from all over the country.
Vanessa Chapman told local news site staff that she helped with the rescue, and could see a person sitting on the overturned boat waving their arms when she arrived.
She said three helicopters took part in the rescue.
Kaikōura is popular with fans of marine life and several local business offer boat trips and helicopter rides to see whales and dolphins.
Mackle told the Associated Press that such accidents are unheard of in the region, but the number of whales in the area in recent days had caused him to worry about a collision.
In 2015, five British tourists were killed in Canada when a whale-watching boat was hit by a wave and capsized.