Dairy industry reassures parents after New Zealand formula poison threat

"Our customers are informed and we do not expect significant business interruption from this threat to the industry," said Penno.

Infant Nutrition Council Chief Executive Jan Carey said infant formula manufacturing in New Zealand was safer than it ever had been, but called on parents to double-check packaging for signs of tampering when buying infant formula.

"Manufacturers are fully aware that they are dealing with a very sensitive product so they have added their own testing regimes on top of world best practice to deliver what is regarded as the most stringent testing processes anywhere in the world," Carey said in a statement.

"They also have security measures designed to keep their products secure and safe through the supply chain. This includes external security, internal locked-down areas, and vetting of personnel who work with the product," she said.

"Mothers can also be assured that New Zealand infant formula manufacturers use tamper-evident packaging to enable broken seals or packaging to be easily spotted."

Police said the letters sent to the Federated Farmers industry group and the Fonterra dairy cooperative were accompanied by small packages of milk powder that subsequently tested positive for the presence of a concentrated form of the poison 1080.

The letters threatened to contaminate infant and other formula with 1080 unless New Zealand stopped using 1080 for pest control by the end of March 2015.

Sodium monofluoroacetate, known as 1080, is a poison used to protect New Zealand's native flora and fauna against introduced pests such as possums and ferrets.

Its use has been controversial over the years with opponents saying it poisons native animals and contaminates the environment.