The threat to infant formula was revealed to the public yesterday because "chatter about it in Auckland" had led to several journalists being tipped off, Mr Key said.
"We always knew there was a risk that it was going to break, I mean, over 1000 people knew," Mr Key told Radio New Zealand today.
Mr Key said the police did not want the threat made public as it could have compromised the investigation.
A reliable test for 1080 contamination also had to be developed.
"There was no test for 1080 because it's not a substance you would find in milk powder, so one had to be created and then the robustness of that tested."
Mr Key told Radio New Zealand the manufacturing process also had to be reviewed.
"And of course the threat in the letter said that we had until the end of March to change our use of 1080."
Meanwhile, police investigating the threat to contaminate infant formula with 1080 poison have appealed today for the person responsible to "do the right thing" and come forward.
Speaking at a press conference at Police National Headquarters this morning, Deputy Commissioner Mike Clement said the police investigation was progressing well.
"We are very encouraged with the response we have had from the public.
"It's most likely with the public's help this matter will be brought to a conclusion."
He asked the person involved to "do the right thing" and make contact with police to prevent further "fear for mums and dads".
"We want to hear from you so we can get this matter resolved."
Dr Pat Tuohy, chief advisor for health from the Ministry of Health, told the press conference he appreciated parents were feeling "anxious and angry".
"They can reassure themselves about the safety of the infant formula chain."
He said formula was as safe today as it was before the threat was made.
Concerned parents could check products for tampering, he said.
"It's not hard to do and can make them feel a lot safer about the formula they are feeding their babies."
He encouraged parents not to change from the formula they were using.
He recommended any parents with ongoing concerns to do as they normally do and seek advice from a health practitioner, such as Healthline, Plunket or their family doctor.
Mr Tuohy said it would it would only take a very small dose to be lethal for a baby.
He said 1080 was not discernible by taste or colour. This would make it more difficult to detect.
However, he continued to assure parents that the likelihood of any poison making its way into infant formula was "tiny".
45,000 samples tested
Food Safety Minister Jo Goodhew said the testing regime did not start until late January because laboratories needed to be geared up with equipment and capabilities to carry out the work.
"We tested the product that we already had on hand, both MPI and Fonterra product, and then as the other manufacturers were contacted we were able to uptake testing all of their products as well," Ms Goodhew told Radio New Zealand.
A total of 45,000 samples had now been tested, including every batch of infant and other formula, and raw milk.
"We're looking at detection to one part per billion of 1080 pesticide, we're testing to that level and we can tell you we've found absolutely nothing across 45,000 samples."
Product samples dating back to September had also been checked, and extra security measures put into the transport and delivery chain.
Ms Goodhew said testing would continue for as long as necessary.
"Health and wellbeing of the consumer is at the forefront of everything we're doing."
Comprehensive testing
Ministry for Primary Industries deputy director Scott Gallacher said all infant formula had undergone a "comprehensive batch testing regime" - from raw milk through to the finished product.
The size of the batch depended on a variety of factors.
"The testing is just one part of the overall security we have across the whole manufacturing and supply chain.
"At each point we have increased vigilance," he said.
Food and Grocery Council chief executive Katherine Rich said she was briefed by the Ministry for Primary Industries in mid-February, and given background on what had occurred and the mitigation steps taken.
However, she told Radio New Zealand she signed a confidentiality agreement so was not able to tell anyone.
Her first move was to check policies on malicious product tampering.
"Most of our member companies deal with these sorts of incidents and plan for these incidents."
- NZME.
