An animal expert says it is unlikely seismic testing caused more than 300 whales to beach themselves at Farewell Spit.
Professor Liz Slooton, of the University of Otago's department of zoology, said it was "remotely possible, but unlikely".
"Apparently the closest seismic survey finished six days ago. I'm not sure about the seismic blasting by the very large ship working off the east coast."
The world's largest seismic blasting ship, the Amazon Warrior, is currently in Hawke Bay, on the North Island's east coast, according to marine tracking data.
However, Slooton said pilot whales move through Cook Straight from time to time and may have been disturbed by that seismic survey.
She said there were regular pilot whale strandings at Farewell Spit, at a rate of almost one per year.
"This is not a new phenomenon. The number of strandings may be increasing due to human impacts, including noise. But strandings at Farewell Spit are not a new thing."
A Greenpeace New Zealand spokeswoman said the NGO had explored the possibility seismic testing had caused the stranding but said it was "unlikely".
She said there was no seismic testing in the area of the stranding, with the last being performed about a week ago.
A report by National Geographic in 2014 said sound bursts, which can reach 250 decibels, can disturb and even harm marine life that rely on sound for communication, navigation, and foraging.
Dr Stuart Hunter of Massey University will perform a necropsy on some of the whales.
"We want to try and determine if there's an underlying reason why such a large number of whales stranded and died in such a short space of time," he said.
"It's not unusual for pilot whales to strand en masse but this stranding is unusual due to the sheer number of whales involved and in such a small amount of time."
He said the goal was to try and understand if there's an underlying reason for the stranding.
Necropsy involves examination of the internal organs of animals to look for signs of infectious disease and trauma.
Today's stranding left more than 300 pilot whales on the sand.
The Department of Conservation said it received a report a pod looked like it might beach at 8pm yesterday.
Department of Conservation ranger Kath Inwood said strandings occur at Farewell Spit most years, with a stranding season between November and March.
The stranding is the third largest recorded whale stranding in New Zealand since data started being collected in the 1800s.
One thousand whales were stranded on the Chatham Islands in 1918 and 450 in Auckland in 1985.