Rescuers tried to help more than a hundred mammals return to sea but many got stranded again. It is relatively common for whales to get stranded on the country's beaches.
More than 400 pilot whales were stranded on a New Zealand beach on Friday, with hundreds already dead as volunteers tried to push the survivors back into the sea, the Department of Conservation said.
It was one of the largest mass beachings recorded in New Zealand, where strandings are relatively common, said Andrew Lamason, a spokesman for the department.
Around 416 pilot whales beached themselves overnight at Farewell Spit in the Golden Bay region at the northern tip of South Island, said Lamason. The Golden Bay area was a "geographical trapping point", he said.
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About 70 percent had perished and attempts were underway to get the remaining whales offshore at high tide, but the outlook is gloomy, he said.
"With that number dead, you have to assume that the rest are in reasonably poor nick as well," he told Radio New Zealand.
"So we're sort of preparing ourselves for a pretty traumatic period ahead."
Pilot whales grow up to six metres (20 feet) in length and are the most common species of whale in New Zealand waters.
Farewell Spit, about 150 kilometres (95 miles) west of the tourist town of Nelson, has witnessed at least nine mass strandings of the species in the past decade.
The latest is by far the largest.
Lamason said the reason the whales beached themselves was unknown but he believed it was partly due to the local geography.
"If you designed something to catch whales then Golden Bay is probably the perfect design," he said.
"Out at Farewell Spit it's a big massive sweeping hook of sand coming about, the bay is very shallow and once the whales get in there... it's very difficult to work out which way is out."