Northland Regional Council says the high number of dairy farmers still breaching resource consents is a reflection of its rigorous monitoring system.
The Sustainable Dairying Water Accord's annual report shows 20 percent of Northland farmers are still breaking the rules for effluent disposal - compared with just 5 percent in many other regions.
The Northland council's regulatory manager Colin Dall said that may be because his council checked every farm once a year - and didn't tell the farmer it was coming.
The figure of one in five Northland dairy farms breaching the effluent rules was still too high - but it was improving year by year.
In recent years the worst offenders have been fined as much as $100,000.