International trading partners first learnt of threats to contaminate New Zealand-manufactured infant milk formula with 1080 poison in February, more than two months after the first letter containing the threat and a sample of contaminated product were received at Federated Farmers' head office in Wellington, in late November.

Prime Minister John Key told a hastily called press conference at the Beehive this afternoon that the government had planned to go public with the threats next week, but that plan was scrapped when news media began to learn of them today.

"Any international market can have 100 percent confidence that the product they receive from New Zealand is safe," said Key.

The decision not to announce the threats earlier was based on police advice and the fact the threat letters gave a timeframe for action. No contaminations were planned until after a March 30 deadline, the date by which the author of the threats wanted the government to ban further use of 1080, which is used to contain pest animal populations that infect dairy herds with bovine tuberculosis and destroy native vegetation and wildlife.

While police characterised the threats as "criminal blackmail", Key said the threats were "a form of eco-terrorism."

"It's the person's motive to try and bully the government to stop using 1080."

Food Safety Minister Jo Goodhew said batch testing of infant formula had been carried out on product manufactured from September onwards, while Key said every fresh batch of infant formula was now being tested for 1080, with none being found in any product.

"We now batch test every single batch that comes off so we know it doesn't have 1080 and all these cans are tamper-proof," he said, while urging consumers to check cans of formula when they bought them. Infant formula manufacturing facilities are subject to high security.

He defended the decision not to go public with the threats earlier.

"The bigger risk would have been to go into the public domain, firstly closing off the opportunity for the police investigation; secondly, closing off the opportunity for us to give reassurance that the products are safe," Key said. "These threats to food internationally are not uncommon and it's reasonably common for no public statements to be made.

"We've made the call to make a public statement because there were queries today. We'd decided we were going to do it next week. We are extremely confident about the manufacturing process. All these products have been tested, so we know these products are safe.

"It is now just a matter of making sure people are careful when they pick up the can at the point of purchase."

The government would continue to support the use of 1080 by the Department of Conservation, in the absence of more effective alternatives to protect native species and reduce the risk of bovine Tb in the national cattle herd and because there would be no negotiation with "eco-terrorists."

New Zealand is unusual for its widespread use of 1080 for pest control, which is fatal to mammals and is unsuited to use in countries that have populations of native mammals. New Zealand has only one such mammal, a native bat.

While native birds can die by eating 1080 poison, their populations have been repeatedly proven to rebound dramatically after application of 1080 in their habitats.

The poison's use is strongly opposed by some recreational hunters and pet-owners who fear their dogs may eat the poison and die and by some environmental activists who do not accept assurances that 1080 does not represent a longer term risk in the natural environment and waterways.

However, a wide range of bodies support its use, including the Royal Forest and Bird Protection Society, Federated Farmers and the Parliamentary Commissioner for the Environment.