Whitebaiters in the Avon river, below the Fitzgerald Ave bridge. Inner-city whitebaiting is popular, but having an impact on the river's health.

Some whitebaiters have been spraying cooking oil into Christchurch's already polluted Avon River to help them fish, a practice that has been condemned by the regional council.

Environment Canterbury (ECan) is urging whitebaiters to stop adding further contaminants to the troubled river, which is widely known to be in a poor state.

The technique supposedly reduces the ripple effect of the water, making the whitebait easier to see. Some whitebaiters also throw rocks at birds to keep them away.

It is understood a whitebaiter alerted Environment Canterbury to the practice.

A group of whitebaiters spoken to by Stuff, who did not want to be identified, said they were aware of the technique but had not done it themselves.

They said it had become less common recently but would not elaborate.

ECan monitoring and compliance manager Marty Mortiaux said the Avon was already over-polluted, and it did not need the burden of cooking oil being added by whitebaiters.

"The reputation of Christchurch whitebaiters has been smeared by a few who are apparently using cooking oil in the Avon river," he said.

"It is taking the gloss off all the work the community is doing to improve the city's waterways, and we definitely don't want this fad spreading to other whitebaiting spots."

He urged whitebaiters to think beyond their own nets and think twice before adding to the river's woes.

Inner-city whitebaiting has become a lucrative activity, and is particularly common along the cracked roads of the eastern central city.

Whitebaiting in the city is legal, but warned against due to the risk of fecal contamination from E coli.

Last month, Canterbury officer of health Alistair Humphrey called for the Avon and the Heathcote rivers to be declared marine reserves, so wildlife in the river could be protected.

In 2014, he described the rivers as "open sewers" and warned people from eating whitebait caught in the city.