The letters contained baby milk formula mixed with highly-concentrated amounts of 1080 poison, and demanded that the country stop using 1080 or he would release poisoned infant formula into the market. There was enough poison enclosed with the two blackmail letters to kill from 13 to 33 infants.
His actions cost the country more than $37 million and sparked a massive police investigation which alone cost nearly $5 million.
By that time, Mr Cox had his own issues with Kerr, who was jailed for eight-and-a-half years in the High Court in Auckland on Wednesday.
"With the belts going well, I told him about other products I had patented - possum pads for joints such as elbows, knees and wrists," Mr Cox said.
"But he went behind my back and started manufacturing and selling them - he cheated me."
Mr Cox hired a patents lawyer to stop Kerr claiming copyright to the products.
"Then one day the police arrived at my door and asked if I would be a witness for the Crown."
Mr Cox's evidence was around the patents dispute over the possum pads and the fact that Kerr was motivated by money.
In his defence, Kerr claimed he sent the threats because he was ethically opposed to the use of 1080.
However, Justice Geoffrey Venning found that he did it for money - Kerr is the owner of another pest-control poison, Feratox, and if 1080 was banned, sales of his ground-laid Feratox poison would rise.
It was, in fact, a visit to Marton in July last year that proved 60-year-old Kerr's undoing.
While staying with a friend, Charmaine Wilson, he sent a letter withdrawing the blackmail threats. The retraction letter was written on a laptop from Nature's Support, and the DNA on the letter was linked to Kerr.
After Kerr's crimes were brought to light, Mr Cox got a phone call from Mahe Drysdale.
"He said, 'You're going to keep the belts going, aren't you?' - he wears them all the time."
And Mr Cox is determined to keep the belts going and also hopes to get his possum pad business up and running.