Image copyright DoC Image caption The only surviving whale was too badly injured to be saved

Fifty-one pilot whales have died after becoming stranded on a beach on the Chatham Islands off New Zealand.

The mass stranding means more than 200 whales have died in separate incidents over the past week in the region.

New Zealand's Department of Conservation says between 80 and 90 whales were found to have become stranded in Hanson Bay on Thursday.

Several dozen managed to refloat themselves but 50 were found dead and one had to be put down.

"There was no likelihood of being able to successfully save the remaining whale. Sadly, the decision was made to euthanise. It was the most humane thing to do," said DOC's Chatham Islands Operations Manager, Dave Carlton.

"This is always an awful decision to have to make."

Image copyright DoC Image caption The dead are being buried close to the beach on the remote island

As is common practice in New Zealand, officials are working closely with local Maori communities to arrange for the handling of the whales.

They are being buried in the sand just behind the beach.

The DoC said skin and blubber samples were being taken for analysis.

The remote and sparsely populated Chatham Islands are about 800km (500 miles) off the east coast of New Zealand's South Island.

The deaths there come less than a week after 145 pilot whales were found dead on a remote beach in New Zealand's far south Stewart Island.

Image copyright DoC Image caption Many of the whales had severe injuries from being caught in the shallow water

Ten pygmy whales were also found dead on a beach in Northland on Sunday.

Whale strandings are not uncommon in New Zealand, but mass incidents are rare and the cause is rarely clear.

Officials say it could be the whales make navigational errors, lose their way in bad weather or while escaping predators, or that they fall victim to illness.