Ten pilot whales died when about 82 of the mammals beached at the top of New Zealand's South Island on Friday, officials said, warning that the others could not be refloated immediately.
"It is an ordeal for them to be exposed stranded like that and be exposed in the sun," Conservation Department spokeswoman Trish Grant said. "Unfortunately some just do die."
"We'd be optimistic that there's surviving whales tomorrow that we can refloat."
The stranding occurred at Farewell Spit on Golden Bay, about 150 kilometres (95 miles) west of the tourist city of Nelson.
Local residents initially reported about 30 whales were stranded and the number grew through the afternoon.
About 100 department of conservation staff were working to keep the whales cool and hydrated in the afternoon sun, Grant said.
The next high tide was due around midnight but it would be too dangerous to refloat the whales then and staff would wait until the morning to carry out the rescue attempt.
Whale strandings are not uncommon in the area and Grant said there were various theories why the animals beached themselves.
"It's something that has occurred reasonably often in Golden Bay with pilot whales, and just even the shape of the bay could mean they kind of get a bit caught with the spit coming round, so it could just be navigational error."
Pilot whales up to six metres (20 feet) long are the most common species of whale seen in New Zealand waters.
Last month 24 died after stranding near Cape Reinga in the far north of the country.
In December 2009, more than 120 whales died in two separate beachings at Farewell Spit and Colville Bay in the North Island.
Explore further: Nearly 60 whales die in New Zealand mass stranding