New Brighton Surf Life Saving Club chairperson Lachlan Hill said a staff member and a surfer had reported seeing a fin in the water at New Brighton Beach just before midday.
It is uncertain what type of shark the fin belonged to, he said, but signage was put up and the water cleared just in case.
Hill said the club's head lifeguard would reassess the situation at the beach on Sunday morning, but he expected the shark had moved on and if safe, the flags would go back up around 11am.
"We don't expect it to be hanging around [for] very long, because we never really see sharks there, and as long as people swim with a buddy and swim between the flags, that's the safest place."
Other beaches across the country were forced to close on Saturday after shark sightings.
Lifeguards at Ōhope near the Top 10 Holiday Park in the Eastern Bay of Plenty evacuated the beach after a shark was spotted in the surf about 4pm.
People could be seen standing on the shoreline looking for the shark before leaving the beach soon after.
Lifeguards took down the flags at Ōhope and blew a whistle to alert swimmers of the shark.
Holidaymakers were also warned not to swim at Whiritoa Beach on the Coromandel Peninsula on Friday after reports of multiple shark sightings.
A spokesman for the Whiritoa Lifeguard Service told the Herald there had been three separate shark sightings close to the shore on Friday and the beach had been closed intermittently.
He said they did not get a good look at the sharks, but thought they may have been bronze whalers - considering how shallow the water was they came into - and said they were "on the smaller side".
He didn't know if the separate sightings were of the same shark.
Surf Life Saving earlier said a shark sighting near the flagged area of Waihī Beach, just down the coast from Whiritoa Beach, forced its temporary closure on Christmas Day.
Shark scientist Riley Elliott told RNZ earlier this week the risk of shark attacks was pretty low.
Over summer, sharks headed inshore to drop off their pups in nursery habitats, he said.
Those areas were usually warm, calm, shallow and had lots of small fish, and generally happened to be the nice places people liked to swim in.
"But what that does mean is we don our Speedos and we go to the beach and we see sharks."
The most common shark people would see in the North Island and Upper South Island was the bronze whaler, Elliott said.
Further south, they could encounter the sevengill shark and the great white shark.
