Environment Canterbury is reminding whitebaiters that detergents are harmful to fish.

The warning comes as ECAN investigates reports of fishermen squeezing cooking oil and washing detergent into rivers.

ECAN principal water quality scientist Adrian Meredith said the practice is illegal and harmful to aquatic life.

"Detergents and cooking oils affect the aquatic life by damaging fish gills, removing the mucus layer on fish, increasing their susceptibility to organic chemicals, things like that."

While the Orari and Avon rivers have been targeted they're not the only ones.

"Particularly where the rivers can be a little bit dirty and therefore the whitebaiters are trying to clear the river so they can see the whitebait and more effectively catch them."

Whitebaiters illegally squeezing detergent into waterways could face fines if caught.

Meredith says they'll be monitoring whitebaiting spots and issuing warnings or fines.