Pollution caused by cow-shed effluent has increased in Northland this milking season, bucking a downward trend in recent years.
The regional council said the number of farmers breaching their resource consents is well up on last season.
The monitoring figures show about 42 percent have been non-compliant, compared with 34 percent last year.
The regional council chairperson, Bill Shepherd, said it had been suggested that could be because of a wet season, and staff layoffs by farmers trying to manage low milk prices.
But Mr Shepherd said the conditions of discharge consents are non-negotiable and excuses are not acceptable.
He said unlike some councils, the Northland council inspected every dairy farm in its catchment.