Torrential or heavy rain can overwhelm wastewater networks by stormwater and groundwater, leading to sewage overflows which drives more diffuse pollution into the ocean.
Some of the beaches with a "high risk" warning in place include: • Mission Bay. • St Heliers. • Judge's Bay. • Milford. • Narrow Neck. • All the East Coast beaches. • All the beaches on and near the Whangaparoa peninsula.
The SafeSwim website, run by Auckland Council, says its models are predicting the levels of faecal bacteria exceed national guidelines at the beaches.
Most water-quality problems at Auckland's beaches stem from animal or human faeces, which can foul water with disease-causing bacteria, viruses and nasty bugs like salmonella, campylobacter and giardia.