WELLINGTON, New Zealand (Reuters) -- Eighty pilot whales have died after stranding themselves on a beach on a southern New Zealand island, a wildlife official said on Wednesday.

Conservation workers and residents are trying to save survivors of a pod of 159 pilot whales that swam ashore on Stewart Island, about 30 km (20 miles) south of the South Island, Department of Conservation spokesman Tom O'Connor said.

"They're pretty distressed...there's no wind and swell to splash on them so they're not in good order," he said.

An attempt will be made to refloat the whales, which look like dolphins and grow up to five metres (16 feet) long and weigh as much as three tonnes, at the next high tide.

New Zealand has one of the world's highest rates of whale strandings, which are thought to occur when the animals become disoriented or when a dominant animal leads others ashore.

But the strandings are not clearly understood by scientists who believe illness or other types of trauma might also play a role.