The stranded whales were discovered by hikers Sunday near Cavalier Creek on Stewart Island, off the southern tip of New Zealand's South Island.
Conservation department staff flew to the area and found that about half of the group were already dead and the others were dying, the agency said in a statement. The whales were well up the beach and the tide was receding, leaving little chance of keeping them alive until more rescuers could arrive.
"Euthanasia is a difficult decision, but is made purely for the welfare of the animal involved to prevent it from prolonged suffering," said Brent Beaven, the official who led the team at the site.
Pilot whales are about 13 feet to 20 feet (4 meters to 6 meters) long and are the most common species of whale in New Zealand waters.
Whale strandings are common in New Zealand. Last month, 24 pilot whales died after stranding on the North Island. In December 2009, more than 120 whales died in two separate beachings near Golden Bay and on the east coast of North Island.