At least 200 whales stranded on a remote New Zealand beach managed to refloat themselves at high tide on Saturday and return to sea, but conservation officials warned the beachings may not be over.

More than 650 pilot whales beached themselves along Farewell Spit at the tip of the South Island in two separate mass strandings over the past several days.

On Thursday, volunteers managed to refloat nearly 100 of the stranded pilot whales.

About 350 whales have died, including 20 that were euthanised.

Hundreds of volunteers from farmers to tourists have spent days at the beach dousing the whales with buckets of water to keep them cool and trying to refloat them.

"People seem to have an emotional attachment to marine mammals," said Department of Conservation spokesman Herb Christophers.

"They've been singing songs to them, giving them specific names, treating them as kindred spirits."

Christophers said everyone is hoping the strandings are finally over, although he said it's possible some of the whales will return to the beach and strand themselves again.

The first group of more than 400 beached whales was found early Friday, with many of them already dead.

"You could hear the sounds of splashing, of blowholes being cleared, of sighing," said Cheree Morrison, a magazine writer and editor who first stumbled upon the whales.

"The young ones were the worst. Crying is the only way to describe it."

Volunteers managed to refloat the surviving whales from that stranding on Saturday, only to hear of a second mass stranding hours later.

Department of Conservation spokesman Andrew Lamason said they were sure they were dealing with a separate pod because they had tagged all the refloated whales from the first group and none of the new group had tags.

In recent days volunteers have formed human chains in the water to try to stop the creatures from beaching themselves again. The volunteers were warned that one of the whales had been found with marks that looked like a shark bite.

Officials will soon need to turn to the grim task of disposing of hundreds of carcasses.

Lamason said one option was to tether the carcasses to stakes or a boat in the shallow tidal waters and let them decompose. The problem with towing them out to sea or leaving them was that they could become gaseous and buoyant, and end up causing problems by floating into populated bays.

Farewell Spit, a sliver of sand that arches like a hook into the Tasman Sea, has been the site of previous mass strandings. Sometimes described as a whale trap, the spit's long coastline and gently sloping beaches seem to make it difficult for whales to navigate away from once they get close.

There are different theories as to why whales strand themselves, from chasing prey too far inshore to trying to protect a sick member of the group or escaping a predator.

New Zealand has one of the highest rates of whale strandings in the world, and Friday's event was the nation's third-biggest in recorded history.

The largest was in 1918, when about 1,000 pilot whales came ashore on the Chatham Islands.