WELLINGTON (AFP) - Rescuers saved 21 pilot whales that stranded on a New Zealand beach but 36 of the animals died, conservation officials said on Wednesday.

The whales came ashore at Ohiwa Harbour in the Bay of Plenty late Tuesday, prompting a major rescue effort involving volunteers and the local Maori community.

The Department of Conservation said 25 of the whales died and 11 had to be euthanised but the rescuers successfully re-floated a group of 21, which swam out of the harbour to deeper water.

Mass pilot whale strandings are common in New Zealand, with scientists unclear about why the marine mammals swim ashore in large groups.