Most people who took the plunge at Northland's popular coastal and freshwater spots last summer were swimming in clean water, according to the latest data.
However, Kerikeri River near the Stone Store and the Hatea River at Whangarei Falls consistently failed quality tests, their popularity with both tourists and ducks largely responsible.
Northland Regional Council (NRC) environmental monitoring manager Jean-Charles Perquin said hundreds of water samples were collected from 44 summer swimming sites between late November last year and late February.
In total 99.9 (606 out of 611) samples at coastal sites and 89.4% (161 out of 180) samples at freshwater sites over summer met national 'guideline values', meaning they were considered suitable for swimming. The water testing looks for bacteria used to gauge the risks of contracting gastrointestinal and other infections.
A report to NRC's recent Environmental Management Committee said water quality was suitable for swimming "at the vast majority of coastal and most of the freshwater swimming sites, either all or most of the time".
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The Stone Store and Whangarei Falls sites had the least suitable water quality, with only 69% and 79% of samples coming within guideline values.
Large numbers of ducks and other water fowl fed by tourists and locals at the Kerikeri spot in particular were the main contributor to its water quality issues.
Microbial source tracking testing had been used at nine sites with consistently elevated bacterial levels, with stock and/or wild fowl markers found at four of them.
The NRC and Whangarei District Councils are working together to address water quality issues at Whangarei Falls caused by factors including contamination by birds and from unfenced stock or faecal-carrying run-off getting into the river upstream.
Results exceeding the guideline usually occurred after moderate to heavy rain.
Weekly results were sent to the Whangarei, Far North and Kaipara District Councils, the Northland District Health Board and other interested parties.
Where results showed elevated bacterial levels within 24 hours it was the health board and/or the appropriate district council's responsibility to take action.
"That could include further testing, public warnings not to swim or gather shellfish and putting up permanent warning signs at the worst sites," Mr Perquin said.
"You shouldn't swim if the water looks dirty or murky, smells or has scum on its surface and also look out for or consider any potential sources of contamination, both nearby and upstream."