By RNZ

A shark expert is urging caution for beachgoers after a suspected shark attack at Waihī beach earlier this week.

Nineteen-year-old Kaelah Marlow died on Thursday after reports she had been injured in the water. If confirmed as a shark attack, Marlow would be the first shark fatality nationally since 2013.

A post-mortem was carried out yesterday, but it is yet to be confirmed whether a shark was behind the attack.

Clinton Duffy, a marine scientist at the Department of Conservation, said many people had the mistaken perception that sharks were uncommon in New Zealand.

"There are actually quite a lot of sharks in New Zealand waters, and large sharks are reasonably common in-shore.

"They can even be very abundant in some places in-shore over the summer," he said.

Duffy said while attacks were uncommon - he counted just 14 fatal attacks since 1840 - people should always swim between the flags, and never alone at a non-patrolled beach. He said people should also avoid swimming at night, and where people are or have been fishing.

He said large concentrations of fish in the water or the presence of dolphins could also indicate the presence of sharks.

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Kaelah Marlow, 19, died in a suspected shark attack. Photo / Supplied

"For the most part, sharks are completely uninterested in humans, I've seen them myself swimming past people ... taking no interest at all," he said.

Surf Life Saving New Zealand said normal patrolling at Waihī Beach had restarted today, following the attack.

Regional manager Chaz Gibbons-Campbell said if anyone saw a shark, they should get out of the water and alert surf life savers.

He said there would be a flagged area for swimmers at Waihī Beach today.

Iwi have imposed a rāhui at the north end of Waihī beach, which remains in place. The collection of seafood is not permitted for a week.

Locals have also been gathering at the beach to place flowers in the water as a tribute to Marlow.