A man who threatened to poison infant milk with 1080 poison has appeared in the High Court at Auckland, pictured.

The businessman charged with blackmailing Fonterra and Federated Farmers has pleaded guilty to both charges against him.

The 60-year-old, whose name is suppressed, appeared in the High Court at Auckland on Thursday after being charged with two counts of blackmail in the Manukau District Court in October.

On Thursday he admitted sending threatening letters to Fonterra and Federated Farmers in November 2014, threatening to put 1080 pesticide in baby milk formula.

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Convictions were entered for both charges but Justice Graham Lang said the factual basis for which the pleas were entered by the accused were in dispute, and a disputed facts hearing was set down for February.

A sentencing date would be decided at that time.

Court documents originally filed at the Manukau District Court said the man was financially motivated.

It was later revealed he was involved with companies that produced pest control products. The names of those companies are suppressed, as is the man's name until at least mid-2016.

Police revealed in March that Fonterra and Federated Farmers had received threatening letters last year, 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 this year. It also threatened to disclose the matter publicly.

﻿1080 is widely used by the Department of Conservation to control pests such as possums, rats and stoats.