Just 30 per cent of popular swimming spots in the Far North are safe for swimming, according to water quality testing over the last three years.
One of the worst sites with a high bacteria risk is the iconic Waitangi Bridge, where children have been leaping into the water for generations. Another site was the river alongside the historic Stone Store at Kerikeri.
The Northland Regional Council's summer water quality testing started this week at 46 popular coastal and 14 freshwater locations across Northland.
But poor results have prompted warnings about overall bacterial risks at 16 out of 23 Far North sites. The results are based on three years' data as a guide to the general water quality.
A total of 12 sites were marked as orange which means "unsuitable for swimming", four are yellow which meant "caution advised" and just seven were green, "suitable for swimming".
Regional council coastal and water quality field operations manager Ricky Eyre said last summer's rainy weather prompted a high number of warnings, particularly at river sites.
This reinforced the need for people to be cautious when swimming after rain, even when the latest results were green.
"With the rainfall, it increases the run-off from land," Mr Eyre said.
"If it's been raining then it's probably a smart move to leave it a day or two to swim, particularly at freshwater sites."
Eyre said the purpose of the water quality testing was to give an indication on whether spots are safe to swim.
The data collected could help lead to some sites being cleaned up, but that was not the purpose of this programme, he said.
Results from the weekly testing will be posted on the "Can I Swim Here?" section of the Land, Air, Water Aotearoa national environmental reporting website (www.lawa.org.nz).
Results will also be forwarded to the Whangarei, Far North and Kaipara District councils, the Northland District Health Board and other interested parties.
Regional council chairman Bill Shepherd said it was the responsibility of district councils to take any further action on elevated bacterial levels.
The areas showing an overall bacteria risk in the Far North (orange) were: Taipa boat ramp, Victoria River at Ratea Forest, Waipapa River at Waihou Valley, Kerikeri River at Rainbow Falls, Kerikeri River at Stone Store, Spinnaker Point in Waitangi, Waitangi Bridge, Tirohonga Stream in Kawakawa, Rawene, Omapere, Waimamaku River and Waipoua River.
The areas showing overall caution of bacterial risk (yellow) were: Tokerau Beach, Coopers Beach, Seaview Rd in Paihia and Te Haumi Beach.
The areas shown as suitable for swimming (green) were: Lake Ngatu, Waipapakauri Beach, Ahipara Beach at Kaka St, Maitai Bay, Matauri Bay, Kororareka Bay in Russell and Opononi.
The summer testing will run until the first Monday in March next year.
Water quality concerns could be reported to the freephone 24/7 Environmental Hotline on (0800) 504-639.