More stranded pilot whales are expected to die as weather conditions and nightfall hamper efforts to save 174 pilot whales that ran into trouble today.
The whales were stranded on Farewell Spit at Golden Bay, near Nelson.
Andrew Lamason of the Department of Conservation (DoC) said up to 500 volunteers could be needed to help get the whales back in the water tomorrow.
Initial estimates of 30 stranded pilot whales rose today to 60, then 143, and finally 198.
Mr Lamason said of those, two dozen had died by 6.30pm and he expected that number to rise.
The Department was scaling down the rescue operation as night approached.
"We can't have people overnight in the water with whales. It's just far too dangerous," Mr Lamason said.
To have a good chance of survival, stranded whales needed overcast or rainy conditions, but Farewell Spit had experienced dry, sunny conditions today.
"It hasn't been a great day to be a stranded whale," Mr Lamason said. "This is a big stranding. It's a real challenge."
He said people had to brace themselves for the unpleasant possibility the pilot whales would have to be euthanised, but rescuers were a long way from giving up on the animals.
"We'll just keep on trying."
The high tide tomorrow evening just before dusk would be a good chance to refloat the stranded pod, he said.
He said most pilot whales in the pod, which included adults and juveniles, were probably closely related.
"It's like a hapu or a village," he said. "It's a whole mixed family group."
Mr Lamason said the geography of Golden Bay meant it was often the site of strandings.
"It's a big, shallow hook. Things come in, they get disoriented, and unfortunately we end up with a lot of dead whales."
Mr Lamason said local iwi would perform karakia on the dead pilot whales.
After that, the creatures would be tethered at mid-tide and nature would "take its course" as happened when three stranded sperm whales were towed to another part of Farewell Spit to decay away from the public in November.
Darren Grover, Project Jonah general manager, was this evening arriving at the site of the stranding.
"We'll be mobilising our supporters from the South Island and ...hopefully from the Wellington region as well."
The whales were about 5-6km east of Triangle Flat at the Spit's western end.
The cause of strandings remained a mystery.
DoC said it was plausible pilot whales' echo-location was not well-suited to shallow, gently sloping waters as they usually preferred steep areas such as continental shelf edges.
"Another theory points to pilot whales' highly sociable behaviour - when one whale loses its way and strands, its pod mates may swim to its aid," the Department said on its website.
Pilot whales are oceanic dolphins and include two species -- long-finned and short-finned.
Mr Lamason said people who wanted to help the pilot whales should arrive at Triangle Flat from 8am to 3pm tomorrow.
He said people should bring wet weather gear, sunscreen, food and water. DoC was installing portable toilets at Triangle Flat.