WELLINGTON, March 13 (Reuters) - The case of two stranded whales saved by a dolphin off the coast of New Zealand could be the first such case in the world, a conservation worker said on Thursday.

Moko the dolphin, a regular visitor to the coast of Mahia on the east Coast of New Zealand's North Island, became an instant hero after leading two pygmy whales that had repeatedly stranded into deep water on Monday.

"As far as I know it's the only documented instance of this happening," said local Department of Conservation officer Malcolm Smith, adding he had checked with whale stranding specialists who were also unaware of any similar dolphin rescues.

Moko, who had been visiting the beach at Mahia on and off over the summer, arrived at the beach in the nick of time, Smith said.

The disoriented mother and calf had resisted attempts to herd them out to sea, and kept restranding on the beach, to the point where Smith said the pair would likely have to be killed.

Then Moko appeared, and came right up to the whales before leading them out to sea.

"Quite clearly the attitude of the whales changed when the dolphin arrived on the scene. They responded virtually straight away," Smith said.

"The dolphin managed in a couple of minutes what we had failed to do in an hour and a half."

Smith said the whales had not been sighted again in the area.

However Moko had returned and was continuing to play with swimmers near the shore, as she has down for about the past six months.

According to Department of Conservation figures about 700 whales strand on New Zealand beaches every year.

The exact reasons why are not known, but theories include sickness, and sloping sandy beaches interfering with whales' sense of direction. (Reporting by Adrian Bathgate; Editing by Jerry Norton)

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