A marine expert has hit out at growing shark hysteria after a popular Kiwi summer spot was closed following the sighting of a 50cm newborn shark.

Surf lifesavers rushed to the water at Waihi Beach yesterday after panicked swimmers reported a shark sighting about 1pm.

Further inquiries confirmed a half-metre long shark had been sighted within the flagged swimming zone on the beach. Lifeguards then closed the beach.

Conservation Department marine scientist and shark expert Clinton Duffy described the move as an "over-reaction".

When asked if he thought shark hysteria had gone too far, Duffy said: "If it has come to the point where people are closing beaches for a half metre, a 50cm, shark, then yes it has.

"The general rule of thumb, that is adopted worldwide, is that any shark above 1.8m should be considered potentially dangerous, if you don't know what sort of shark it is.

"When they are getting to around 1.5 to 1.8m long, some of the species are getting big enough to inflict serious injury if they were to bite you."

Duffy initially did not believe that Waihi Beach had been closed due to the sighting of what would have been a baby shark.

"Half a metre, it wouldn't even be able to get its mouth around your kneecap," he said. "It might just be able to bite you on the toe. Half a metre . . . that is newborn. Good grief."

A Waihi Beach Surf Lifesaving Club spokeswoman said they were first alerted to the shark after "some girls came in and started panicking to us about something that was swimming with them".

She said other swimmers told them they had spotted "a baby one [shark] . . . about half a metre long or something like that".

Lifeguards then decided to close the beach, making a series of announcements over a speaker system telling beach-goers to get out of the water. While the shark was "tiny", the decision to close the beach had been made out of fear it could have been accompanied by larger sharks.

"It was a tiny shark, but we just decided to do it just in case it was in a pack," she said.

But Duffy said it was very unlikely that such a small shark would travel in a school with sharks of the size capable of inflicting serious injury or death.

Most sharks formed schools with sharks of a similar size to themselves.

"Most species of sharks that school segregate by size and sex," he said.

"So if that shark was part of a [school], they would all be half a metre long.

"It would be death by 1000 cuts."

Beachgoer Lukas Davidson said about 200 people were at Waihi Beach when lifeguards took the action.

He said people were calm throughout.

"We saw people move out of the water pretty quickly," he said.

"It's a really nice, bright, sunny day, so a lot of families are about . . . there are a lot of young people enjoying the beach. So when it happened, a lot of people moved out of the water pretty quickly."

It is the second time in recent weeks that Waihi Beach has been closed after a shark sighting. Last month there were three reported shark sightings there, with the beach being closed on Christmas Eve when a shark was spotted outside the patrol flags.

Earlier in the week Pauanui Beach, on the Coromandel, was closed after eight to 10 sharks were spotted close to the shoreline.

The beach had been closed twice previously in recent weeks.