A dolphin has jumped into a boat off New Zealand's North Island, critically injuring a woman in what was probably a world first according to experts.

The 27-year-old woman was struck by the leaping dolphin near Slipper Island in the western reaches of the Bay of Plenty yesterday afternoon.

She was admitted to Auckland Hospital's intensive care unit in a serious condition. The dolphin was unharmed.

Dolphin expert Alan Baker today described the incident as a freak accident.

"I've been studying dolphins for 40 years and this is the first time I've heard of a dolphin jumping into a boat," he told National Radio.

"More often a boat hits a dolphin rather than a dolphin hitting a boat.

"This is, I think, a unique occurrence and a most unfortunate one indeed."

Dr Baker was not aware of the exact circumstances of the incident, but said dolphins were very acrobatic animals so if a boat was travelling at speed that could have been a contributing factor.

"They jump for various reasons — courtship, just announcing that they're in an area to other dolphins — and if the boat was travelling at speed and came across a pod that was jumping a dolphin could be caught unawares and just end up landing in a boat," he said.

Dolphins normally had very good judgment and were often seen in close proximity to boats, he said. The one involved was possibly a bottlenose, which can grow up to four metres in length.

"They do a lot of jumping and twisting and turning and flopping and carrying on," he said.