Nearly 200 pilot whales stranded themselves on New Zealand's South Island on Friday, and hordes of rescuers rushed to the remote area in a bid to guide them back to sea.

Two dozen of the 198 whales had already died despite efforts to save the creatures, which were found stranded on Farewell Spit, a famous spot for whale beachings, Department of Conservation area manager Andrew Lamason said.

About 80 conservation workers and volunteers were trying to refloat the whales as high tide rolled in, Lamason said. But with such a vast number of animals stranded, workers were bracing for days of arduous work to move the whales back into the water.

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And even if the whales were refloated, that was no guarantee they would survive, Lamason said.

"We've had plenty times in the past where the pods have gone out to sea and turned around and come back again," Lamason said. "We're preparing for a big few days."

Pilot whales grow to about six metres long, and large strandings are common during the New Zealand summer. Experts describe Farewell Spit, located on the northwest corner of South Island, as a whale trap due to the way its shallow waters seem to confuse whales and diminish their ability to navigate.