On the day Fonterra received a package of its own infant formula laced with the poison 1080, the Government convened its top committee for dealing with national security threats.

The Officials Committee for Domestic and External Security Coordination (Odesc) set in motion a chain of events that pulled in 1000 people to track the source of a likely hoax, and shore up New Zealand's food safety system before the inevitable day when the Government would have to go public.

From the Prime Minister and senior Cabinet Ministers, dairy executives, diplomats, Crown Research scientists, police and security officials, as well as supermarket heads and employees, they were all aware of the threat.

Aware of the potential economic consequences, each kept it quiet for four months, while the Government pulled together a media strategy.

But authorities did not know then, and they still don't know, whether they were dealing with an extremist "agri-terrorist", or an introvert with a grudge and too much time - police said they were still hoping it was the latter.

On Tuesday, it was revealed that Fonterra and Federated Farmers received threatening letters last November, along with milk packages that tested positive for the poison.

The letters threatened to contaminate infant formula and other products if New Zealand did not cease to use the poison by the end of March. It also threatened to disclose the matter publicly.

That was four months ago, and while police said they had "many persons of interest", they conceeded they had only spoken to a limited number.

"We've been introspective in our approaches," Police Assistant Commissioner Mike Clement said.

"What we were endeavouring to do before we went public, was to carry out the investigation in a way out of the public eye, which meant not approaching those people."

On the side of the Government, the priority was to balance consumer safety with New Zealand's dairy-dependent economy.

While the blackout period allowed the police to carry out their investigations covertly, it also gave the Ministry of Primary Industries (MPI) time to create a new test for 1080 in milk and begin the process of retrospectively testing milk product batches from as far back as September, and every batch made since.

Security was stepped up along every step of the supply chain, but MPI have confirmed it was only in recent weeks they alerted global infant formula companies, grocery distribution companies and retailers.

From the day it was publicly announced supermarkets pulled products from their shelves, restricting them to being purchased from behind the counter or in places covered by CCTV.

Meanwhile, Federated Farmers was conducting site visits as early as December to ensure Fonterra processing plants had stepped up security adequately.

Around this time, the Ministry for Foreign Affairs had begun briefing diplomats who were passing information to key countries including China, the United Kingdom and the Middle East.

Prime Minister John Key said he made the decision very early on that the public would be informed of the incident - against the advice of the international trading community.

"In the end, over a thousand people knew, the person themselves said they were going to go public, but putting all of that to one side I would just pose the question that if we didn't go public beyond that point when the person who sent the threat [made the deadline], then I think New Zealanders would say 'that's a coverup'", Key said.

The plan was to go public next week, but that was brought forward to Tuesday - the day journalists caught wind and began asking questions.

With the economy heavily dependent on New Zealand's $14 billion dairy export industry, an adverse market reaction could have had significant consequences.

‎Rabobank International director of dairy research, New Zealand and Asia, Hayley Moynihan said the markets had remained relatively calm - a vote of confidence in New Zealand's systems.

But the test would be at the next global dairy auction next Tuesday.

Similar food safety threats were not uncommon overseas, and in many cases were never notified.

Moynihan said the global food industry did recognise these things happened from time-to-time.

Trade Minister Tim Groser said New Zealand had done all it could, to protect the economy from any negative fallout.

"We're engaged in a very constructive discussion with our trading partners; giving them a mix of information, some of it highly technical.

"I think ultimately the decision rests in the hands of our trading partners so we can't pre-empt them," he said.