A group of 215 pilot whales have died after they became stranded on a remote New Zealand island, environmentalists say.

Many of the whales were already dead when they were discovered in the northwest corner of Chatham Island, some 840 kilometres east of mainland New Zealand, environmental charity Project Jonah said.

"The Chatham Islands is a challenging spot for stranding response, known for great white sharks, remote beaches and a resident population of less than 800 people," the charity said on Facebook on Saturday.

Project Jonah said those factors meant the whales that were still alive had to be euthanised.

"While we always hope surviving whales are able to be refloated, this wasn't an option here."

Whale strandings are not uncommon on the Chatham Islands, with the largest event recorded in 1918 when more than 1000 animals died in a single stranding.

Source: Australian Associated Press