About 500 pilot whales have died in mass strandings on New Zealand's remote Chatham Islands, the government said Tuesday after ruling out a rescue effort in the area's shark-filled waters.

Two "super pods" of the dolphin-related cetaceans beached on two islands in the remote chain and survivors were euthanised, according to the Department of Conservation.

On Friday, 250 beached pilot whales were found at Chatham Island, and then three days later another 240 were reported on Pitt Island, the government said.

The locations -- far from the New Zealand mainland -- made a rescue operation impossible, authorities said.

"Due to the risk of shark attack to both humans and the whales, the surviving whales were euthanised by our trained team to prevent further suffering," Dave Lundquist, a government technical marine advisor, told AFP.

"This decision is never taken lightly, but in cases like this it is the kindest option."

The bodies will be left to decompose naturally on site.

Mass strandings are not uncommon on the Chatham Islands, where the largest recorded beaching involved an estimated 1,000 whales in 1918.

In 2017, there was a mass stranding of almost 700 pilot whales.

Scientists do not fully understand why mass strandings occur, but some researchers think pods go off track after feeding too close to shore.

Pilot whales -- which can grow to more than six metres (20 feet) long -- are highly sociable, so they may follow pod-mates who stray into danger.

Around 300 animals beach themselves annually in New Zealand, according to official figures, and it is not unusual for groups of between 20 and 50 pilot whales to run aground.

But numbers can run into the hundreds when a "super pod" is involved.

Last month, almost 200 pilot whales died on a beach in Australia's remote western Tasmania. State wildlife services managed to refloat 44 of the mammals.