The swimming alerts currently in place for Auckland's beaches.

Almost 40 beaches in Auckland have been deemed "high risk" after weekend rain.

According to Auckland Council's Safeswim database, 39 city spots posed the threat of causing illness from swimming.

The warnings came after a Sunday filled with rain for the city and on Sunday night, a black alert, indicating a very high risk of illness from swimming, was issued for the North Shore's Castor Bay.

The beaches affected are mainly on the North Shore and include Orewa, Army Bay and Browns Bay.

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Safeswim, which launched in 2017, and is designed to encourage swimmers to check the waters first before swimming, provides a real-time forecast covering more than 80 beaches and freshwater locations around Auckland.

Browns Bay has had issues with E coli levels making it unsafe to swim for the last month.

High levels of E coli can cause severe abdominal cramps, diarrhoea and vomiting.

Auckland Council and Watercare placed a temporary public health warning on the bay last week.