More than 400 pilot whales have become stranded at Farewell Spit in Golden Bay.
The Department of Conservation (DOC) said about 70 percent of the 416 whales that stranded had already died.
It has put out an urgent call for volunteers to help staff keep the remaining whales cool until high tide at 10.30am.
DOC spokesman Andrew Lamason said it was the biggest whale stranding the department's Takaka office had ever seen.
"The guys will be out there at the moment trying to keep the whales as calm as possible," Mr Lamason said.
"We won't go for a general push [to re-float] until water's all around the whales."
It was a big high tide, which would help, he said.
However, staff were very concerned for the surviving whales.
"With that number [already] dead you can assume that the others are in pretty poor nick as well."
People who wanted to help should come to the base of Farewell Spit as quickly as possible, he said.
"Bring your own food, own water, own clothing."
People just wanting to rubberneck should stay away, Mr Lamason said.
Project Jonah general manager Daren Grover said about 40 medics will be on-site helping volunteers.