"We let the health board and relevant district council know of any results showing elevated bacterial levels, typically within 48 hours, and it's their responsibility to take any further action needed," NRC chairman Bill Shepherd said.
Where contamination is found, further tests and site investigations can be done to establish the source, with warnings of issues and signage up telling the public not to swim or gather shellfish at the worst sites.
Shepherd said last summer 88.7 per cent of samples at coastal sites and 56.6 per cent at freshwater sites over summer met national "guideline values", meaning they were considered suitable for swimming.
"Results can vary quite considerably from year to year, largely due to whether we experience a dry or relatively wet summer. Last year we had a wetter summer and this was reflected in our results, which were generally worse than normal, especially for coastal sites."
In contrast, during the drier summer of 2016-17, 99.8 per cent of samples at coastal sites and 92.9 per cent at freshwater sites over summer met the guideline values.
The advice is always for people not to swim for two or three days after heavy rain, which can carry run-off from land, or if there are other indications the water is unsafe.
"As a rule of thumb, don't swim if the water looks dirty or murky, smells or has scum on its surface and look out for or consider any potential sources of contamination, both nearby and upstream," Shepherd said.
The summer testing will run until the first Monday in March next year. Water quality concerns can be reported to the freephone 24/7 Environmental Hotline on (0800) 504 639.
