WELLINGTON (AFP) - More than 50 pilot whales have perished in another mass stranding in New Zealand, taking the death toll of beached marine mammals to over 200 in the past week, wildlife authorities said on Friday (Nov 30).

A pod of 80 to 90 whales were spotted late on Thursday on the shore of remote Chatham Island, about 800km east of the South Island, the Department of Conservation said.

It is the fifth stranding in New Zealand in less than a week, including 145 pilot whales which all died after they beached last weekend at Stewart Island, off the southern coast of the South Island.

By the time rangers reached the latest group of animals early on Friday, 50 had died, one remained stranded but alive, and the rest had re-floated themselves and returned to sea.

Department of Conservation manager Dave Carlton said the surviving whale was in poor condition and was euthanised.

"It was the most humane thing to do. This is always an awful decision to have to make," he said.

Chatham Island was the scene of New Zealand's largest recorded stranding in 1918, involving 1,000 whales.

The reason whales and dolphins strand is not fully understood, but factors can include sickness, navigational error, geographical features, the presence of predators, and extreme weather.

In addition to the strandings at Chatham and Stewart Islands this week, a group of 12 pygmy whales also beached in New Zealand's far north, and a sperm whale and a pygmy sperm whale grounded themselves in separate incidents on the North Island.