The 2014 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.
His actions cost the country more than $37 million and sparked a huge police investigation.
By that time Mr Cox had his own issues with Kerr, who was jailed for 8 years in the High Court at 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," he 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.
Mr Cox is determined to keep the belts going and hopes to get his possum pad business up and running.