Most Otago waterways are safe to swim in - most of the time, a report card for last summer shows.
Nationally, 57% of 206 monitored freshwater swimming spots during the 2009-10 summer had water quality that met guidelines almost all the time, a Ministry of the Environment report said.
The guidelines required at least 95% of the samples to have safe ecoli levels.
However, 11% of the sites breached the guidelines regularly and were unsuitable for swimming.
Of the eight Otago sites in the report, the Kakanui Estuary received the worst rating, the second lowest, with only 75% to 90% of samples complying with guidelines, At three sites, two on the Taieri River and the Manuherikia River at Galloway, 90% to 95% of samples complied and the Waikouaiti at Bucklands had 100% compliance.
The other three sites were not sampled enough to get a rating.
The Otago Regional Council, which provided the figures for the ministry, also put out its own water-quality report card.
Council environmental information and science director John Threlfall said the report showed of 97 samples taken at freshwater sites during the summer, six did not meet the guidelines.
After rainfall it would be unadvisable to swim in any waterways, he said.
Three failed samples from the Kakanui, Pomahaka and Taieri Rivers came after rainfall from January 6-11.
"Rain washes contaminants from stock off land.
Even reasonably low intensity farming can cause pollution problems with ecoli in a river catchment after rain."
Of the 54 estuary and marine water samples taken, four did not meet the guidelines, two (Waikouaiti and Macandrew Bay) came after rainfall and two (Waikouaiti and Kakanui) did not.
The Kakanui River at Clifton Falls exceeded the guidelines three times, with significant rainfall before each failure, the report said.
Its estuary also failed a sample but not due to rainfall.
Investigation of previous years' failures on the river had not shown the cause of the problem.
The council planned weekly, instead of monthly testing this summer in a bid to find the cause.
The council was addressing the water quality issue with proposed changes to its water plan, he said.
