Sydney Harbour’s glistening, sapphire-blue water has turned a muddy brown because of storm water washing soil and debris into it.

       The drastic change in the colour of the water was caused by overflowing wastewater systems, with authorities saying it could linger for days. Officials have also urged swimmers to stay away from the harbour until the situation improves.

       The New South Wales (NSW) environment department issued pollution warnings for all Sydney beaches, in areas from Palm Beach in the north down to Cronulla in the south.

       “Due to extreme wet weather conditions and flooding events, stormwater pollution may be impacting some swimming sites,” the department said.

       “Avoid swimming if you can see signs of pollution such as discoloured water and floating debris. Always check with lifeguards for the latest beach conditions.”

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       The warning came in the aftermath of record-breaking floods and torrential rain that have affected the country’s eastern coast since last month and killed at least 21 people.

       Sydney residents who witnessed the sudden change in colour were left shocked, and took to social media to share photos.

       “This is Sydney Harbour this afternoon. Other passengers and I pretty taken aback,” Twitter user Carly Waters said, sharing a drone video.

       Several compared the appearance of the harbour to the Brisbane River, in the Queensland capital, and Melbourne’s Yarra river, which are often brown in colour because the rivers carry rainwater, which dredges up clay and sediment.

       The murky colour of the waterways along the New South Wales coastline was caused by flood water, sediment and untreated sewage, with the potential for viruses and bacteria to pose a health risk to swimmers.

       “It’s not just the local storm water that’s polluting our beaches, but it’s the murky water from the flood-affected inland areas,” said Meredith Campey, the manager of the NSW government’s Beachwatch programme, which provides pollution forecasts. “It’s flowing downstream into the ocean.”

       She said places like Pittwater, Sydney Harbour and Botany Bay were the worst affected, as lower levels of flushing meant storm water discharging into the ocean was trapped in estuaries.

       Ms Campey said the dangers from the polluted water could linger for days due to the extent of the floods.

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       Earlier this week, the appearance of the famous Hyams Beach on the south coast of New South Wales shocked beachgoers after they saw its white sand turned red with “maggot-infested” seaweed, something experts said could be the result of deadly storms lashing the country.

       Images showed the beach covered with dark red algae as visitors described noticing a “mild smell”.