"I got on quite well with him at the start, but I didn't know what sort of guy he was. I'm a practical person - he was a bit tricky."

That is retired Whanganui farmer Colin Cox's assessment of Jeremy Kerr, the blackmailer who was jailed this week for threatening to poison infant milk formula with 1080.

Mr Cox, from Fordell, is the inventor of a possum belt that relieves back pain, and in 2013 he struck a deal with Kerr to manufacture the belts.

Kerr set up a company, Nature's Support, in Marton's Oxford St, to make the belts and, with Olympic champion rower Mahe Drysdale endorsing the product, business was good.

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Mr Cox said he had never discussed 1080 with Kerr and knew nothing of his involvement with blackmail letters sent to Fonterra and Federated Farmers.

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.