Fifteen pilot whales died on Sunday after a mass stranding at Port Levy on Banks Peninsula, near New Zealand's South Island city of Christchurch.

The whales have been stranded on the beach at around 06:30 a.m. on Sunday local time, New Zealand Press Association reported.

New Zealand Department of Conservation personnel and local people managed to return some whales to the water, and about 35 have survived.

Project Jonah chief executive Kimberly Muncaster said mass strandings in this part of the country were very unusual.

She said saving whales is not as easy as it looks and one of the most important aspects of such a mission is top make sure no- one tips water into a whale's blowhole or obstructs the opening in any way.

Muncaster said they would continue to monitor the whales for some time to make sure they do not re-strand, and some volunteers would spend the night on the beach keeping an eye on the dead whales.

Scientists will take samples on Monday to try and establish what caused the stranding, and the dead whales will be buried after consultation with local Maori iwi.

More than 120 whales died over 48 hours in two separate beachings in New Zealand in December last year.

About 20 of them died at the beach of Colville Bay, north of Coromandel township.

Meanwhile, 105 long-finned pilot whales died at Farewell Spit at the top of the South Island.