Fonterra has been forced to dump 150,000 litres of milk because 14 tankers were potentially contaminated with mud and gravel.
The dairy co-operative says the contamination happened at its Hawera plant on Friday, when a contractor's tanker came in for cleaning with 1-2 kg of mud and gravel inside.
The material made its way into the cleaning system, which reuses water and detergent for multiple vehicles.
Fourteen other tankers were cleaned using the potentially contaminated water before an automated system raised the alarm 44 minutes later.
By that time, six of the tankers had picked up 150,000 litres of raw milk, which had to be dumped at a waste treatment plant.
Fonterra says it does not yet know where the mud and dirt originally came from.
The latest problem for the company comes after the report of an independent inquiry, released this week, was critical of Fonterra's handling of its botulism scare in August.
Fonterra general manager for the lower North Island Scott Walls says the company is investigating the source, including reports it may have come from gas and oil drilling waste.
He told Morning Report the findings of the inquiry won't be known for at least a week.
Mr Walls says the incident was minor and the company's systems quickly alerted staff to the problem. He says the company quickly isolated the affected tankers.
If the investigation throws up problems with Fonterra's systems, improvements will be carried out in the company's other plants, Mr Walls says.
The latest problem for the company comes after the report of an independent inquiry, released this week, was critical of Fonterra's handling of its botulism scare in August, saying a host of mistakes and errors of judgement were made.