A 60-year-old Auckland businessman has been arrested over threats to poison infant milk formula, bringing an 11-month long investigation to an end.

Police Commissioner Mike Bush told media this afternoon that the man will be appearing in Manukau District Court this afternoon facing two counts of criminal blackmail.

READ MORE: 1080 threat - man in court

The charges relate to letters sent to Fonterra and Federated Farmers in November last year threatening to poison infant formula if New Zealand did not stop using the pest-control poison by the end of March this year.

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The public was informed of the threat in March and infant formula was pulled from the shelves of supermarkets and held securely to prevent contamination in store.

If found guilty, the maximum sentence for the charges the businessman faces is 14 years imprisonment.

Five search warrants were executed this morning in Auckland Rangitikei District, Mr Bush said.

"The arrest follows a long and complex investigation.

"It is one of the biggest investigations police have undertaken in recent times and reflects how seriously we view crimes of this nature," Mr Bush said.

"Today's development sends a clear message to how seriously we take threats of this criminal nature and how we will use every resource no matter how long it takes to bring threats of this nature to a conclusion."

A team of 35 officers worked on the investigation, dubbed Operation Concord, and were assisted by specialist staff, Mr Bush said.

Police considered more than 2600 people in relation to the threats and 60 people were identified as of significant interest and were approached for interviews.

The total cost to police to date was over $3 million, Mr Bush said.

Chief executive of Ministry for Primary Industries Martyn Dunne said the positive conclusion to the investigation could put people's minds at rest.

"We have indeed... a world class food safety system," he said.

The testing regime on infant formula would continue, Mr Dunne said.

More than 150,000 batch tests have been carried out since the threat.

He had not thought as far ahead as whether civil lawsuits could be pursued by MPI or companies, Mr Dunne said.

"Certainly I would assume the reaction from the industry would be relief," he said.

Police Minister Michael Woodhouse said the arrest was a milestone "in what has been a long and complex investigation over the last 11 months".

"The investigation is one of the most significant undertaken by police in recent times," he said in a statement.

"Having visited the Operation Concord investigation team in Auckland I can attest to the commitment and tireless efforts which have led to today's arrest, and I want to thank them for their efforts."

The Minister said he was not able to comment further on the case now that it was before the courts.

Prime Minister John Key said the threat had been extremely serious, despite the remote risk that it would be carried out. He was told late last night that an arrest would be made.

"It is very good news that the police have followed up...obviously I can't say too much because it is before the courts, but it will be a great relief if that is the individual in question.

"Because there would still be mothers who would be very, very concerned about the implications [of a 1080 contamination]."

Labour leader Andrew Little said it sounded as though the task facing investigators had been akin to looking for a needle in a haystack, and they should be congratulated.

"If they have got the right person and they do secure a conviction, then good on them for any amazing effort, for what was a plan that could have had huge implications for New Zealand industries long term."

Fonterra chief executive Theo Spierings said the co-operative was pleased to hear an arrest had been made.

"We would like to thank MPI and the New Zealand police for their continued and exhaustive efforts on behalf of the New Zealand dairy industry, retailers and the general public.

"We'd also like to acknowledge the significant joint industry response to this threat."

Foodtsuffs managing director Steve Anderson, whose company owns the Pak'N Save and New World supermarket brands, was also pleased with the arrest.

"There is now a legal process to be followed and as such we are unable to comment further. But I can confirm that our stores will continue to remain vigilant while the police work through this process."

Timeline of the threat and response

• November 27, 2014: Fonterra and Federated Farmers receive a letter threatening to contaminate infant formula.

• On the same day a watch group of government agencies is formed to consider the threat. The Prime Minister, the Minister for Primary Industries, the Minister for Food Safety, the Minister of Police, the Minister of Foreign Affairs, and the Minister for Trade.

• January 15, 2015: Milk testing methods validated, and the sampling strategy approved.

• February 10: First meetings with affected infant and other formula manufacturers.

• February 11: Initial meetings with major global suppliers.

• February 13: Food and Grocery Council chief executive told of the threat.

• February 18: Initial meetings with New Zealand supermarket chains.

• February 25: Labour leader Andrew Little told of threat.

• March 10: Public announcement at media conference.

• October 13: Arrest made.

- additional reporting Isaac Davison