Police are investigating threats that infant formula has been contaminated by 1080 poison.

At a press conference at the National Police Headquarters in Wellington this afternoon, police say they launched an investigation in November and all testing so far has revealed no contaminated formulas.

More than 40,000 raw milk and product samples were tested and there was no detection of the poison.

Blackmailers have threatened dairy giant Fonterra over the use of 1080 poison in pest control.

Fonterra and Federated Farmers were sent packages of powder in November, which was later identified as the fatal dose of the poison.

The letter threatened to contaminate the formula unless New Zealand stopped using 1080 for pest control by the end of this month.

Police are appealing for anyone with information about the threat to contact police.

They are also asking for the people or the person who made the threat to come forward.

An Auckland-based investigation team - Operation Concord - involving up to 36 staff at different times, has been working closely with the industry and the Ministry for Primary Industries to identify those responsible.

"Whilst there is a possibility that this threat is a hoax, we must treat the threat seriously and a priority investigation is underway," says Deputy Commissioner (National Operations) Mike Clement.

"We therefore encourage the letter writer to come forward to Police. The letter writer may not have really considered the implications of their actions when this communication was drafted. Now is the time to put this right by picking up the phone and calling us.

"At present a large number of resources are being directed to this multi-faceted investigation which involves a number of specialist groups and support agencies.

"We are pursuing a number of lines of inquiry, and are also at the point where the public's help is being sought."

More than 40,000 raw milk and product samples were tested and there was no detection of the poison.

Anyone with information should contact: 0800 723 665, email: opconcord@police.govt.nz, or call anonymously via Crimestoppers on: 0800 555 111.

New Zealand has a diary industry worth $11 billion.