Beachgoers at Takapuna are angry there are no signs warning people about recent pollution that forced lifeguards to clear the surf last night.
The North Shore beach is subject to a temporary "no swim" warning after stormwater flowed into the water, although Auckland Council can not order the beach be closed.
Lifeguards shut down the beach at 3.30pm yesterday, when a fault caused a stormwater drain to overflow.
Lifeguards initially believed the contamination was raw sewage, but a Watercare spokesman has since confirmed it was not a wastewater overflow, and was not related to Watercare's network.
After a heavy downpour of rain, foamy water had flowed from a stormwater outfall, and tests of the water did not show signs of sewage contamination, the spokesperson said.
The Auckland SafeSwim website continues to regard the beach as high risk, saying there is an "elevated health risk due to poor water quality; temporary no-swim warning in effect", although water quality was expected to be fine by this evening.
At least 30 people, mainly children, were in the water when the Herald visited the beach just before midday.
Tasigrace Pace, 18, from Otahuhu, said her family found out about the pollution issue after the children had gone in to swim.
"If the kids get sick or anything, then we will definitely take it up with council," Pace said.
Priya Rajagopalan, a visitor from the United States, said her family was told by an elderly couple about the risk.
Rajagopalan, a mother of two children aged 4 and 14, said the family had originally planned to spend half a day at the beach.
"The kids have been terribly disappointed. Why are there no signs telling us about what happened or warning about the risks?"
Local resident Albert Tong said lifeguards were asking people to get out of the water yesterday afternoon. But things were back to normal this morning.
"I assumed everything has been resolved."
Lifeguard Dan Lee said a sign warning people of the water quality was put out at noon and will remain until 7pm.
"Our advice is for people not to get into the water, but at the end of the day it's still really up to them."
Martin Neale, technical lead for Auckland Council's Safeswim system, acknowledged there had been a "little bit of a gap" where no signage had been set up on the beach.
That was because the signs were put out by and managed by surf lifeguards, who didn't start patrolling until midday.
A large digital sign was planned to be installed at the beach by the end of the month.
But Neale said the Safeswim website was designated to be peoples' primary source of information.
"We've spoken to colleagues in Melbourne who have told us that once people have made the effort to drive to a beach, they are going to go in the water anyway, so what we are trying to do is capture people before they drive there."
Static signs had nonetheless been placed around beach sites for people who didn't use the internet, but not at beaches like Takapuna that were patrolled by lifeguards.
Takapuna was one of a number of Auckland beaches that are considered unsafe for swimming.
The website provides water quality forecasts and information about health and safety risks at 84 beaches around Auckland.
It lists a number of beaches around the area including other popular North Shore beaches and others on Waiheke Island as being unsafe because of poor water quality.
For more information visit www.safeswim.org.nz.