Health authorities are urging people to check the quality of their favourite beach or swimming spot before diving in this summer.
Auckland Council has assessed the quality of more than 180 beaches and freshwater sites in the region ahead of the holiday season and found that only a few are unsafe to swim at.
They are Wairau Outlet, Cox's Bay, Meola Reef, Weymouth Beach and Little Oneroa Lagoon.
However, heavy rain could lead to run-off into waterways and people are encouraged to avoid them after large downpours.
An Auckland Council spokesman said people should also note signs warning against swimming and avoid swimming in high-risk areas such as stormwater outfalls and stream mouths.
Almost 70 Auckland sites will be checked over summer, although the water quality at some beaches - including Devonport, Omaha, Maraetai and Kendall Bay - is good enough that monitoring isn't required.
In Waikato, the water quality at many monitored river sites remains poor, including in the Waikato River at Horotiu Bridge, Huntly Bridge, Mercer Bridge and Tuakau Bridge.
The quality at all coastal beaches in the region is considered good.
Most monitored sites in the Bay of Plenty are at present safe to swim in, including all lakes except Lake Okaro.
Toi Te Ora Public Health Service medical officer of health Dr Jim Miller said people needed to take a "reasonable approach" when using the water.
Council websites offered a guide, but the results weren't necessarily definitive.
"It means you should exercise caution and see whether the water is clear or smells."
Health effects from swallowing water tainted with faecal micro-organisms or other bacteria can be unpleasant, and include diarrhoea or vomiting, and infections of the eye, ear, nose and throat. Children are particularly at risk of ear and skin infections.
Dr Miller said swimmers who contracted stomach bugs generally recovered after a few days of resting and drinking fluids, "but if it doesn't clear up, or it's seeming to get worse, it's certainly a good idea to seek medical advice".
However, he believed the biggest risks were safety related.
"Take advantage of the lifeguards, swim between the flags and it's obviously not a good idea to go swimming after you've been drinking," he said.
"If you're going boating, follow the safety codes and take a life jacket."
• Check the quality of your local swimming spot here.
Auckland swimming spots: The good and the grim
• Goat Island Marine Reserve, Leigh
• Anchor Bay, Tawharanui Regional Park
• Kendall Bay, Birkenhead
• Cheltenham Beach, Devonport
• Sunkist Bay, Beachlands
• Omana Beach, Omana Regional Park
• Okoromai Bay
• Wairau Outlet (permanent health warning)
• Cox's Bay (permanent health warning)
• Meola Reef (permanent health warning)
• Weymouth Beach (permanent health warning)
• Little Oneroa Lagoon, Waiheke