Nearly 200 pilot whales stranded themselves on New Zealand's South Island on Friday, and hordes of rescuers rushed to the remote area in a bid to guide them back to sea.

At least 24 whales from the pod of 198 that beached themselves at Farewell Spit had died, and rescue workers were trying to refloat the survivors, the Department of Conservation (DOC) said.

"Re-floating stranded whales is a difficult and potentially dangerous job,” DOC spokesman Andrew Lamason said. “Community group Project Jonah has 140 volunteers in the Golden Bay area who are trained to do this, and we’re working alongside them."

If the re-float attempt late Friday failed, then the rescuers would have to wait 24 hours for another high tide before trying again, Lamason said.

Farewell Spit beach, at the northern tip of the South Island, has been the scene of many mass pilot whale strandings over the years. There have been at least eight in the past decade, including two within the space of a week in January last year, although the latest stranding is one of the largest.

Scientists are unclear why they strand themselves in large groups, with some speculating that healthy whales beach themselves while trying to help sick or disoriented family members that are stranded. Others believe the topography of certain places, such as Farewell Spit, somehow scrambles the whales' sonar navigation, causing them to beach.

Once stranded, the whales are vulnerable to dehydration and sunburn until rescuers can use the high tide to move their massive weight back into deeper water. Once refloated, the whales often simply swim back ashore and have to be euthanized.

Pilot whales grow up to 20 feet long, weighing up to 3 tons, and are the most common species of whale in New Zealand waters.

Wire services