Fifty-one pilot whales have died after becoming stranded on a remote island in New Zealand.

It comes less than a week after 145 of the whales died in similar but seemingly unrelated circumstances in the country.

This latest incident saw up to 90 of the mammals beaching themselves at Hanson Bay in the Chatham Islands, over 800km east of the mainland.

By the time conservationists arrived, 40 had managed to free themselves.

However, another 50 had died and one, who was found alive, had to be euthanized due to its condition.

New Zealand Department of Conservation workers clear dead pilot whales in the Chatham Islands, 30-11-2018. Image: New Zealand Department of Conservation

Stewart Island stranding

Last week’s incident happened on Stewart Island – a small island off the south of the country’s South Island.

Some 85% of the island is a protected nature reserve. When a hiker came upon the 145 beached whales, they were half buried in the sand and around half of them were already dead.

The rest were in very poor condition and had to be euthanised as their remote location would have made it impossible for rescuers to reach them in time.

On Sunday meanwhile, 10 pygmy killer whales were found stranded at Ninety Mile Beach on the North Island.

Some 200 volunteers managed to refloat eight of them – however they stranded themselves a second time on Wednesday and had to euthanised.

We've been getting a lot of questions about why whales strand.

Strandings are natural and have been occurring for millennia. There are many theories, but in many cases the cause is unknown and unlikely to be due to any one factor alone.

Read more: https://t.co/fxl9vWFjh8

— Department of Conservation (@docgovtnz) November 28, 2018

Beached

Marine expert Dr Dave Lundquist said there is no evidence confirms the incidents are linked.

"You're talking about strandings across the entire breadth of New Zealand in a very short period of time, which naturally does cause everyone to reflect on whether those might have something to do with one another," he said.

He said scientists there is no scientific consensus on what causes whale strandings to occur – noting that there are probably a range of reasons.

New Zealand Department of Conservation workers clear dead pilot whales in the Chatham Islands, 30-11-2018. Image: New Zealand Department of Conservation

He said they could be caused by whales navigating incorrectly, trying to escape from predators, or some of them suffering injuries or illness.

He said there could also be man-made factors like underwater noise.

"In many of those cases, it's probably a combination of those factors," he said.

Whale strandings are most common in New Zealand during the southern hemisphere spring and summer.