Fonterra has been fined $24,000 for allowing nitric acid and a caustic cleaning agent to be discharged into a storm water drain and a water course - the fourth such conviction in the Bay of Plenty this month.

The giant dairy co-operative was successfully prosecuted by the Bay of Plenty Regional Council in Tauranga District Court for the chemical spill at its Edgecumbe milk processing factory .

Langford Holdings, of Waihi Beach, Kildarragh Farms, of Rotorua, and Edward Tye, of Katikati, were also convicted for discharging dairy effluent onto land and fined $13,000, $12,000 and $26,500 respectively.

The council's water management manager Eddie Grogan said the convictions were a timely reminder to holders of resource consents that they must be compliant at all times.

"All three dairy cases were a result of poor management of effluent systems, including ponds and irrigators. Farmers need to ensure maintenance of their equipment is undertaken regularly, and that staff are fully trained in the operation of the equipment," he said.

The council had been working closely with Fonterra, Federated Farmers and DairyNZ to help farmers achieve compliance, he said.

Nitric acid is a highly corrosive and toxic acid which is often used in fertilisers.