The E. coli contamination of popular Taranaki swimming holes has saddened the area's medical officer of health, Jonathan Jarman.
Swimming spots in King Edward and Windsor Parks on Patea River in Stratford have been off limits since January 7.
Dr Jarman said he was upset that swimming in the river has become a health risk.
"I'm always saddened when popular swimming holes are out of bounds because of faecal contamination because swimming at the river is such an iconic Kiwi summer activity.
"It's wholesome family fun. The children have a great time plus it's free."
He said people risked contracting tummy bugs, skin irritations, ear infections and coughs and colds from swimming water with high levels of E coli bacteria, which is used as an indicator of of faecal contamination.
Last year, Taranaki Regional Council traced similar contamination in the Patea river to cows, but all upstream dairy effluent discharges were compliant with their consents.
The council lifted a similar health warning at the popular swimming holes on the Waiwhakaiho River at Merrilands Domain in New Plymouth at the weekend.
Dr Jarman said there had been no reports of people becoming ill after swimming in river water this summer. On average the local DHB had about seven cases notified to it each year.
"When the district councils started putting out warning signs that meant the risk of becoming ill was 5 per cent which meant one in 20 people [were] likely to acquire an illness from swimming at that particular site," Dr Jarman said.
"One in 20 doesn't sound a lot but let's say you have 1000 people and it's a really busy spot. Then what that means is 50 people will get sick."
Looked at from a population health point of view Dr Jarman said it was a significant problem.
Council investigating water quality at Patea River
The council said it tested recreational swimming sites about once a week in summer, and staff used their judgement on whether to carry out further testing or investigation.
The council said it was looking into what caused the elevated E. coli levels in the the Patea River in Statford.
It said in a statement a high concentration of the indicator bacteria meant that it was more likely that disease-causing organisms were present, but did not mean that anyone swimming in the water at the time would actually be affected.
"Water quality in our rivers and at our beaches is generally pretty good over summer, except in poor weather conditions.
"Heavy rain flushes contaminants from urban and rural land into waterways and we strongly advise people not to swim for at least three days after heavy or prolonged rainfall - even if a site generally has good water quality."
Other water warnings in Taranaki
Warnings remain in place in Taranaki at Lake Rotokare inland from Eltham where cyanobacteria was at an "unacceptable risk" level and at Lake Rotomanu, the Waiwhakaiho and Te Henui river mouths in New Plymouth.
The regional council said problems at Rotokare were likely due to natural conditions of the lake - warm water temperatures, nutrients from the native bush and the absence of flushing through the lake.
Large numbers of water fowl were blamed for elevated levels of E coli at Lake Rotomanu and the Waiwhakaiho and Te Henui river mouths.
- Robin Martin