Fonterra and Federated Farmers received blackmail letters in November 2014 containing threats to contaminate formula with 1080 unless New Zealand ended its use of the poison by the end of March.
Biodegradable 1080 is the salt form of fluoroacetate, a toxin found in several plants.
1080 pellets are applied aerially in New Zealand to kill rats, stoats and possums. The latter pest has been blamed for the spread of bovine tuberculosis in New Zealand.
Samples of milk powder sent with the letters tested positive for a concentrated form of 1080.
Federated Farmers, which lobbies on behalf of its farmer members, and Fonterra referred the matter to New Zealand Police, which has been working with the Ministry for Primary Industries (MPI) to identify the individual behind the threats.
"Whilst there is a possibility that this threat is a hoax, we must treat the threat seriously and a priority investigation is underway," said Mike Clement, deputy commissioner, New Zealand Police.
"We therefore encourage the letter writer to come forward to Police. The letter writer may not have really considered the implications of their actions when this communication was drafted. Now is the time to put this right by picking up the phone and calling us," he said.
Despite the threat, MPI insists that New Zealand-made infant formula is "as safe today as it was before this threat was made."
Scott Gallacher, deputy director general, MPI, said the "ability of anybody to contaminate infant and other formula during manufacturing is extremely low."
"Regardless, we encourage people to be vigilant when buying infant and other formula. Our advice is always to check packaging for signs of tampering. We are reinforcing that advice as a result of this blackmail threat."
"Security and vigilance has been significantly increased since this threat was received."
He added that more than 40,000 samples of raw milk and milk products have been tested negative for 1080 since the threat emerged, with no positive results.
MPI and Fonterra have also established a "validated testing regime that is being used by the dairy industry for both raw milk and dairy products targeted by the threat."
"Immediate and decisive steps"
Tankers of raw milk arriving at Fonterra plants have been tested for 1080 since mid-January.
All paediatric products and nutritional base powders manufactured by Fonterra since September 1 2014 are also being tested, the company confirmed.
"We can fully assure our customers and consumers that all of our milk products are safe and of high quality, and our supply chain continues to be secure and world-class. We are playing our part in helping the Government manage the criminal threat, as is the rest of the dairy industry," said Theo Spierings, CEO, Fonterra.
"We have taken immediate and decisive steps to give our customers and consumers added confidence - including testing and security measures," Spierings added.
Dr William Rolleston, president, Federated Farmers, said the steps taken by MPI and Fonterra have created "formidable barriers to any such threat being carried out."
In a statement, Synlait Milk, which produces Synlait brand infant, follow-on and growing-up formula and a2 Platinum products for the a2 Milk Company, said it is "confident that its food safety systems and security standards protect the integrity of its products."
Westland Milk Products, New Zealand's second largest dairy cooperative, said "there is no evidence that the safety of its products has been compromised."