Swimmers should stay out of the water in areas where people are fishing to avoid sharks, a marine scientist says.

The warning comes after Safeswim on Thursday advised people not to swim at Ōmanu Beach and Papamoa Beach, both in the Bay of Plenty, after shark sightings.

A week ago, a near one-metre-long suspected bronze whaler was spotted by lifeguards at Ōmanu.

Auckland Museum curator of marine biology Clinton Duffy said bronze whalers were common and usually not aggressive, but people should avoid swimming in areas where people were fishing.

"They'll come for either the burley or the bait, it's either the bait or the struggles of the fish, and that can turn a normally sort of benign species, like a bronze whaler, into quite an aggressive animal."

People should leave the water quickly and quietly if they spot a shark, he said.

But shark sightings at popular beaches were common at this time of year and are usually no cause for alarm, Duffy said.

"The bronzies are in close to the beaches this time of year, they come in around about October. Pupping continues all through the summer and then they hand around in close inshore to chase flat fishes, sting rays and eagle rays and feed, basically."

Surf Lifesaving New Zealand Eastern region manager Chaz Gibbons-Campbell said it was not unusual to see sharks so close to shore at the time of year.

-RNZ