A number of beaches on Auckland's North Shore are closed on Tuesday due to pollution.
A wave of foamy stormwater flushed out through Takapuna Beach at 3:30pm on Monday, forcing lifeguards to pull the flags.
Auckland Council's Safeswim website currently has the patrolled beaches at Takapuna, Milford, Mairangi Bay, Browns Bay and Long Bay listed as high risk for swimming.
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Raw sewage flowed directly into the water between the red and yellow flags at Takapuna Beach on Monday afternoon, Surf Life Saving Northern Region (SLSNR) said.
However, Watercare says the event was not related to its network and tests done on Tuesday afternoon do not show signs of sewage contamination.
"Though our lifeguards will be there, they will have flags down and they'll be warning people the water is not safe," said SLSNR operations manager Adam Wooler.
"Obviously if people do go in, they do so at their own risk, we can't prevent people from going in. However, our recommendation and the council's is to stay out."
Mr Wooler said the incident highlights wastewater problems at a number of Auckland's beaches.
"We have had to respond [to bad water quality] a number of times already this summer by dropping the flags."
The Safeswim website says the affected beaches will be closed for at least Tuesday morning and afternoon.
Spokesperson Dr Martin Neale says 31 Auckland beaches are currently at high risk.
"There's an elevated risk of infection at 15 beaches due to yesterday's rainfall," he says.
"There are also 16 that have long term warnings, as they breach international guidelines."
Dr Neale says the near 40 millimetres of rain that fell in just a few hours yesterday is what washed out so much contamination.
On the west coast, Aucklanders seeking swim at Piha or other popular beaches are being warned of huge swells and strong currents.
"Big surf produces a lot of high energy rips that carry people out to sea," said Mr Wooler.