“It is international best practice to avoid swimming for 48 hours after heavy rain,” the Watercare spokesperson said.
Rain could send a “first flush” of dirty water through stormwater drains, onto beaches and into urban waterways.
Along for the ride could be animal faeces, oil, metals, rubber from tyres, microplastics and rubbish.
This morning, Safeswim deemed the beaches along Tāmaki Drive from Okahu Bay to St Heliers off-limits because of contaminated wastewater overflow. Orakei Basin and Te Tinana were also off-limits, as were Herne Bay and Home Bay, on the other side of the CBD.
Yesterday, the downtown Karanga Plaza Harbour Pool in the Viaduct Harbour was considered unsuitable for swimming but the alert had been lifted today.
A handful of beaches on Auckland’s North Shore including Narrow Neck, Milford and Castor Bay were also contaminated.
According to Safeswim, when a sensor detects an overflow on Auckland’s wastewater network a black pin automatically appears at the affected swimming spot for 48 hours.
The spokesperson said a red rating meant modelling indicated a high risk of getting sick from swimming.
Run-off after heavy rain was one factor that could make the water unsuitable.
A green rating indicated a low risk of illness.
Safeswim combined continuous monitoring of Auckland’s wastewater network with modelling to forecast water quality at individual swimming spots, said the spokesperson.
The service updated forecasts every 15 minutes.
Digital signs displayed the latest advice at beaches including Browns Bay, Takapuna, St Mary’s Bay and Herne Bay. Swimmers could also check their chosen beach’s water condition on the Safeswim website or app before heading out.
Tom Eley is a multimedia journalist at the Waikato Herald. Before he joined the Hamilton-based team, he worked for the Weekend Sun and Sunlive. He previously worked as a journalist at Black Press Media in Canada and won a fellowship with the Vancouver Sun.