Rescuers who tried desperately to refloat a huge pod of beached whales were to return at first light today to see how many survived the night.
The Department of Conservation said about 416 pilot whales stranded near the base of Farewell Spit, Golden Bay overnight on Thursday.
Some 250 to 300 were already dead when the whales were discovered early yesterday morning, sparking claims DoC should have acted sooner.
The mass stranding is the third largest recorded in New Zealand since the 1800s.
Takaka DoC operations manager Andrew Lamason said rescuers were due to leave the beach about 8.30pm last night, as any of the whales appeared "very distressed" and "slowly slipped away".
Volunteers taking part in the mass rescue mission tried to refloat the whales yesterday and kept them cool with buckets of water. Collingwood Area School students also helped out, some singing a waiata to the pod.
Earlier in the day 100 of the whales were refloated at high tide, but about 50 returned to the beach and restranded themselves.
Lamason said the refloated survivors swam in the wrong direction and headed back into the bay.
"There are about 50 [refloated] whales [remaining] offshore, but they're not looking great out there, just milling around.
"If you were going to design a geographical trap for whales, Golden Bay is pretty much perfect."
About 80 to 90 whales were still alive on the beach last night, as more than 500 volunteers helped in yesterday's rescue effort.
Professor Liz Slooton, of the University of Otago's department of zoology, told the Herald there was a wide range of causes for strandings.
Whales may beach themselves because they were sick, dying, giving birth or disoriented.
While earthquakes and storms could be a factor, human causes, including noise, may lead to a whale beaching itself, Slooton said.
Slooton added it was "remotely possible, but unlikely" that seismic testing had caused the mass stranding, as suggested yesterday.
"With a mass stranding like the one at Farewell Spit - usually only one or two individuals are sick or in trouble."
She said the rest of the pod would not leave the area because whales had "very strong social bonds".
Pilot whales are normally found in water several hundred metres deep, and often more than 1000m deep, Slooton said.
The layout of the Farewell Spit, an area known for strandings, did not help.
"Pilot whales - also sperm whales and beaked whales - are not very familiar with shallow water.
"The large areas of very shallow water near Farewell Spit will be confusing for several reasons. It will be difficult for the whales to figure out which way they need to swim to find deeper water as it's shallow over such large areas."
On a "normal" stretch of coastline the whales would not have to go far to find where the water gets deeper, she said.
"Echolocation doesn't work all that well in shallow water. Rather than bouncing back nicely - like off a wall - the sound will tend to bounce away from the whales, with a much smaller proportion of the signal bouncing back."
DoC ranger Kath Inwood said in previous whale strandings the carcasses have been either buried in the dunes, or tethered and taken to sea to naturally decay.
Specialists from Massey University would conducted necropsies on some of the dead whales yesterday.
Minister of Conservation Maggie Barry said it was "terribly sad to see these magnificent creatures in this state".
The Interislander ferry had offered free passage on its afternoon sailings for marine mammal medics headed to the rescue operation.
The stranding is listed as the third largest in New Zealand recorded history, after 1000 whales were stranded on the Chatham Islands in 1918 and 450 in Auckland in 1985.
A deadly season for whales
The deadliest months for whales beaching themselves at Farewell Spit is from November to March, according to DoC.
Stranding "hotspots" include two beaches on the Chatham Islands, the Coromandel, Northland, Kaipara and Stewart Island.
Recent Farewell Spit strandings include;
February, 2015: 98 pilot whales stranded.
January, 2014: About 70 whales strand themselves.
January, 2014: Nine whales are euthanised after beaching themselves twice.
November, 2012: 28 pilot whales stranded at the high-tide mark.
January, 2012: More than 80 whales die after a mass stranding of 99 pilot whales.