About 36 pilot whales that had become stranded in the Eastern Bay of Plenty in New Zealand have died.

Two whale pods beached themselves in the Ohiwa harbour on the north-east coast on Monday.

Wildlife conservationists launched a rescue operation and helped one pod to be refloated on Tuesday, while 21 more whales were successfully herded out to sea on Wednesday.

The reasons for mass pilot whale strandings are not well understood.

Pilot whales are particularly prone to stranding behaviour. The largest known pilot whale stranding involved an estimated 1,000 whales at the Chatham Islands in 1918, according to the DOC.

Steve Brightwell from the DOC said the whales came into Ohiwa harbour after one of them was unwell and beached itself, reported Radio New Zealand.

Eleven of the whales were euthanized on welfare grounds on Tuesday, the Department of Conservation (DOC) said. About 25 pilot whales were found dead early Wednesday morning.

'Great effort'

At about 05:00 local time on Wednesday (16:00 GMT Tuesday) volunteer medics from Project Jonah, a non-profit organisation, along with the DOC began an operation to guide those whales that were still alive back out to sea.

Those pilot whales still alive were guided through the harbour to the ocean
"We did it! All 22 whales were shepherded through the mouth of the harbour to the ocean and were last seen heading towards deeper water," Project Jonah said on Facebook.

New Zealand on average has more whales stranding themselves than any other country in the world, Daren Grover, general manager of Project Jonah told the BBC.

"It's something we have lived with and we are quite geared up to respond to," he said.

"Today was a great effort - all those that were alive were refloated."

The reasons for mass stranding are not well understood
Scientists believe individual whales strand themselves because they have a disease and are coming to the end of their life.

However, there are numerous theories around mass stranding including their highly sociable behaviour. One theory is that as one whale becomes stranded the other members of its pod try to help and become stranded themselves.

Pilot whales are the largest member of the dolphin family. They get their name from the fact that researchers believe that each pod follows a "pilot" in the group.

Their distinguishing feature is a large bulbous forehead, which protrudes beyond the mouth and small beak.
Volunteers who have been trained as medics sprang into action on Wednesday
Scientists think individual whales beach themselves when they are reaching the end of their life