Fonterra says it has completed its operational review, which has found reprocessing after plastic was found in whey protein at its Hautapu plant in the Waikato led to the contamination event.
"An item of non-standard equipment (used during the reprocessing) caused the contamination," said Fonterra strategy director Maury Leyland.
At a press conference this afternoon chief executive Theo Spierings said the recall "was the right thing to do", despite there being no presence of botulism-causing bacteria in the whey protein.
"We are very much in recovery mode," Spierings said.
He said the contamination, which occurred in May last year, was not escalated fast enough.
"The red flag should have gone up faster."
Fonterra now needed to focus on becoming "best in class" in terms of food safety, he said.
"The (dairy) season is kicking in and we need to get into action mode."
Spierings said the plastic had entered the whey protein as a result of human error.
"It just dropped in," he said.