Horizons Regional Council's summer swim spot monitoring programme is under way to help inform the public about potential health risks.
The annual monitoring programme runs from November to the end of April and tests bacteria levels for freshwater rivers and lakes, and coastal beaches.
Some freshwater sites are also regularly monitored for potentially toxic algae.
Horizons collects, analyses and reports on swim spot monitoring over summer through the Can I Swim Here? module on the Land, Air, Water Aotearoa (Lawa) website to help the public decide when and where they can swim over summer.
"Weekly water samples collected from each site are sent to an independent accredited lab for testing," Horizons natural resources and partnerships group manager Dr Jon Roygard says.
"Results are received within 48 hours and are updated on the Lawa and Horizons websites, where interactive maps show each of the swim spots marked by a red, amber or green location marker to indicate that week's results."
The traffic light system used is part of the Ministry of Health and Ministry for the Environment guidelines that provide guidance to councils and district health boards on how they should act when certain levels of indicator bacteria or algae are detected.
The public and their animals should stay out of the water if they are at a river and see black or dark brown, slimy mat-like growth on the stones that may also be musty smelling.
"We do want to remind people that water quality is just one factor in a number of potential health risks," Roygard says.
"We ask the public to be aware of hazards such as unstable banks and cliffs, submerged logs, and tsunami warnings and rips at beaches.
"Another thing to remember is that as sampling is weekly, the results may not always reflect the water quality for the whole week, especially if it has rained. A general rule is if the water looks clear and it's three days after rainfall, you should be good to go."
Roygard says the results from the monitoring programme are used to help inform Horizons' policies and the work programmes to enhance water quality.