A shark expert says there's very little risk of sharks taking on swimmers in New Zealand - as they are in New South Wales. 

Four attacks over the past two days have left two people in hospital with critical leg injuries. 

Auckland Museum Marine Biology Curator, Clinton Duffy, says they're by bull sharks, which are one of the only species big enough to attack humans. 

He says New Zealand's climate is not appropriate. 

"The waters would have to warm up an awful lot before bull sharks would make home here."

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