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The dead whales were found beached in two areas of Farewell Spit, a strip of sand on the country's South Island.

Rescuers managed to refloat another 15 of the mammals and are monitoring their progress toward safer waters.

The 15 dead whales ranged in size from small calves to six metres (20 feet) long, according to Conservation Department (DOC) ranger Nigel Mountfort.

He said the remaining 15 whales were not yet out of danger because they remained in a tidal area where they risked beaching themselves again.

A conservation worker has been sent on a plane to monitor the bay for whales at risk of further stranding, he said.

DOC spokeswoman Trish Grant said: "It's still happening, we don't know what the outcome is going to be at this stage and there is still definitely a concern they will strand.

"It all depends on which way the whales start to move."

It was also coming up to low tide, which was a high risk time, she said.

Scientists are unable to explain why whales strand, but some believe it is caused by disorientation in their sonar sounding systems.