Takapuna Beach was made subject to a temporary "no swim" warning after stormwater ran into the water.
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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.
The researchers aim to create early detection tests, improving survival rates.