Pool owners have been warned not to empty pool water directly into stormwater drains which can kill freshwater ecosystems.
The Waikato Regional Council said it has investigated several cases of chlorinated water going into the stormwater system and ending up in streams where it killed eels and other aquatic life.
Staff recently responded to an incident in the Hamilton suburb of Melville where swimming pool water was being discharged into the stormwater system, rather than the wastewater system, which sent the water travelling into a nearby stream.
"It's that time of the year when people are cleaning their pools in preparation for the hot weeks ahead," the regional council's incident response team leader, Wayne Reed said.
"Chlorinated water poses a threat to freshwater ecosystems. So we strongly urge pool owners to check their systems are correctly plumbed to send pool water into the wastewater treatment system and not into the stormwater system."
It is a breach under the Resource Management Act to discharge chlorinated water and the council said those responsible can be fined.
The council has urged owners to check their pools are correctly connected to the wastewater treatment system.