Police did not say what species of shark was involved in the attack. Clinton Duffy, a shark expert with the Department of Conservation, said New Zealand is a hotspot for great white sharks, and other potentially lethal species also inhabit the waters.
Attacks are rare. Duffy estimated that only 12 to 14 people have been killed by sharks in New Zealand since record keeping began in the 1830s.
"There are much lower levels of shark attacks here than in Australia," he said. "It's possibly a function of how many people are in the water" in New Zealand's cooler climate.
He said that during the Southern Hemisphere summer, sharks often come in closer to shore to feed and to give birth, although that doesn't necessarily equate to a greater risk of attack.
"Ninety-nine percent of the time they ignore people," he said. "Sometimes, people get bitten."
Around the world, sharks attacked humans 80 times last year, and seven people were killed, according to the University of Florida's International Shark Attack File. The death toll was lower than it was in 2011 but higher than the average of 4.4 from 2001 to 2010.
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