Waikato Regional Council says dairy farmers in the region appear to be picking up their game, with half the number guilty of nitrate pollution compared with this time last year.
Compliance manager Rob Dragten says the council has started its annual monitoring of farms, to check on whether farmers are breaching effluent management rules.
He says the first in a series of helicopter flights found one in ten farms were significantly non-compliant - compared to one in five farms last yaer.
But Mr Dragten says the most serious cases appear to be worse, mainly due to over-full effluent storage facilities.
He says the council will investigate all farms found to be significantly non-compliant, to see whether it needs to take further action.