Twenty eight pilot whales have died after stranding in a sandbar at the mouth of Okuru River, south of Haast.
The Department of Conservation (DOC) was alerted to the stranding on Wednesday afternoon.
Of the 38 whales found, ten whales are alive, DOC spokeswoman Trish Grant says.
Ms Grant says when found, the whales appeared to have been there for around 12 hours.
"We are assessing whether it is safe and feasible to re-float the whales around high tide about 2pm this afternoon."
She says the sea there is very dangerous with strong currents so it is not safe for people to be in the sea refloating the whales.
Project Jonah general manager Daren Grover says they are expecting to find more dead whales upon assessment this morning.
He says Haast hasn't had a mass stranding in recent times, but mass strandings were common all over the New Zealand coastline.
In February last year, more than 400 whales were stranded in Golden Bay. Nearly 75 percent of them died.
The largest stranding ever recorded in New Zealand was 1000 pilot whales which beached in the Chatham Islands in 1918, followed by 450 of the same species on Great Barrier Island in 1985.
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A series of shallow earthquakes struck within 15 minutes near the Southern Alps on Tuesday, two near Hokitika and one near Methven.
Mr Grover says while earthquakes can affect whale pods, mass strandings usually have more than one cause.
"Connections have been made with undersea quakes - they make sound waves and these animals live in this environment of sound, so it can cause them to move, to flee to coastal waters they are less familiar with," Mr Grover says.
"But we often find with strandings it's not one reason - it's a combination of factors. It could be they were closer to shore foraging with a larger swell or a weather front pushing them closer to shore, or an orca nearby caused them to flee to a tidal area which eventually led to the stranding."
DOC staff, Makaawhio iwi members and local people cared for the whales and are returning to the area this morning to look for any that may have moved overnight and assess the situation.