Takapuna Beach was made subject to a temporary "no swim" warning after stormwater ran into the water.
Lifeguards shut down the beach at 3.30pm yesterday, when a fault in the network caused a stormwater drain to overflow.
The flags were taken down and the beach was closed for swimming for the remainder of the day - with the water quality labelled "high risk".
Takapuna was one of a raft of Auckland beaches that are currently unsafe for swimming.
The website provides water quality forecasts and information about health and safety risks 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 due to poor water quality.
For more information visit www.safeswim.org.nz.