Swimmers were told to get out of the water at Muriwai Beach near Auckland today after a report of a shark.
Lifeguards believe it was a false alarm and may have been Maui's dolphins that were swimming in the area, which was the site of a fatal shark attack last Wednesday.
Muriwai Volunteer Lifeguards chairman Tim Jago said a board rider reported seeing a small shark, about 1-2m long, at Maori Bay, next to Muriwai, about noon.
Swimmers and surfers were asked to get out of the water while lifeguards deployed a boat to investigate.
Other board riders then told lifeguards they had seen two Maui's dolphins, the world's rarest and smallest known subspecies of dolphin, in the same area a short time earlier.
"We saw nothing more so we are working on the basis that it was due to everyone's heightened sense of vigilance or anxiety," Mr Jago said.
"We're so comfortable that there's nothing wrong out there that we've got school children on an organised tour back in the water."
Muriwai local Adam Strange, 46, was killed by what is believed to have been a large great white shark as he swam at the beach last Wednesday.