Around 300 whales beach in New Zealand every year, with the largest documented event being the stranding of over 1,000 in 1918 on the Chatham Islands.

This is one of the worst incidents involving beached whales in the country's history.

There are renewed fears the rescued whales will beach themselves again as they attempt to stay with their stranded fellows in the bay, he added.

Numerous more than 100 whales refloated off the South Island's Farewell Spit today at high tide have become stranded again, reports Radio New Zealand. About 50 of the whales swam out, while 80 to 90 re-stranded on the beach.

Volunteers have been working in collaboration with the department and marine mammal charity Project Jonah since Friday morning to return surviving whales to the ocean.

Project Jonah said there is no one single reason why whales become stranded, and possible explanations range from disease to extreme weather.

More than 70 per cent had died by sunrise on Friday morning but conservation staff and local volunteers were trying desperately to save the remaining 100.

The majority of the work is keeping the whales wet and cool, while rescuers wait for high tide.

Volunteers somehow had managed to save the lives by re-floating some of the whales during high tide, but a lot of them were quickly re-stranded as the tide ebbed as reported by local media reported on Friday.

Magazine writer Cheree Morrison discovered the beached whales as she tried to capture the sunrise with a photographer and guide.

Over 500 helpers came to get the rest of the whales into the waters before they too expired.

The second largest was in Auckland in 1985, when 450 ended up on a beach. "The most likely hypothesis is that pilot whales' echolocation is not well-suited to shallow, gently sloping waters, because they generally prefer high relief (steep) areas such as the edge of the continental shelf", according to a DOC fact sheet.

Project Jonah reports New Zealand has one of the highest whale stranding rates in the world.

It's not entirely clear why whales strand themselves - theories range from distraction while following prey to attempts to protect sick pod members, or even disorientation caused by irregularities in the magnetic field - but pilot whales are especially prone to mass strandings due to their exceptionally social nature, according to the American Cetacean Society.