Almost 100 pilot whales have died in a mass stranding on New Zealand's remote Chatham Islands, conservation officials said Wednesday.

Most of the marine mammals beached themselves over the weekend but rescue efforts were hampered by the area's isolated location, about 800 kilometres (500 miles) east of the South Island, the Department of Conservation (DOC) said.

Department biodiversity ranger Jemma Welch said 69 whales had already died by the time wildlife officers reached the beach.

She said 28 pilot whales, including two that beached on Monday after the initial stranding, and three dolphins were euthanised.

Welch said the animals had to be put down "due to the rough sea conditions and almost certainty of there being great white sharks in the water which are brought in by a stranding like this".

She said members of the local Maori community had performed a ceremony to honour the spirits of the whales, which would be left to decompose naturally.

The Chatham Islands was the site of New Zealand's largest recorded mass stranding, when 1,000 beached themselves in 1918.

Pilot whales grow up to six metres (20 feet) long and are the most common species of whale in New Zealand waters.

The causes of mass strandings remain unknown despite scientists studying the phenomenon for decades.

Theories include pod members following a sick leader ashore, shoreline geography that scrambles the animals' sonar, the presence of predators and extreme weather.