The man behind threats to spike milk formula with 1080 was snared after police found his DNA on a “retraction letter”.

Jeremy Hamish Kerr, 60, of Mellons Bay pleaded guilty in December to two counts of blackmail over the fiasco told police he “just cracked”.

The hearing, which started yesterday, arose because the defendant disputed some of the Crown’s allegations.

In an interview with police in October Detective Senior Sergeant Aaron Pascoe told Kerr a letter sent to police retracting the threats to contaminate milk formula with 1080 contained trace DNA 260 times more likely to belong to Kerr than any other random male.

“I find that absolutely staggering. Amazing,” the defendant said.

Kerr later came clean about being behind the plot.

“I had bad mental health and just cracked one day, to be honest,” he said.

“The moment I did it, I regretted it.”

But Kerr immediately denied he was financially motivated but triggered by a media story he saw about 1080.

“I’ve been involved in the industry so long, you see some things that are not good,” he said.

“I didn’t want to see [1080] banned. I just wanted to see it used more responsibly.”

Mr Pascoe revealed data on his laptop showed Kerr had tried to print a letter to a police officer who had previously questioned him.

The letter, later received by police, questioned their handling of the situation.

“You were aware there was no threat of any sort made to New Zealanders of 1080 in milk formula. I can’t imagine why the government put the country through this,” it read.

Kerr wrote he had had enough of the “nonsense” and had destroyed the 1080.

“I wanted to make it go away out of my life,” he told Mr Pascoe.

The Crown says Kerr was the creator of a pest-control product - Feratox, an alternative to 1080 - and that his motivation for the crime was financial.

Christine Gordon, QC, said had 1080 been banned, Feratox would have been the “natural replacement” and would have seen a spike in usage to control the possum population.

With the defendant obtaining 10 per cent in royalties from each sale, that would put him in line for a large windfall.

Ms Gordon said Feratox sales were on the slide and the income was not enough to live on.

At the time the threats were sent, Kerr's credit card was almost maxed out and he could not make the minimum repayments, she said.

In a police interview, he downplayed the issues saying it was his royalties totalled $100,000 a year and he was "not in any stress".

Kerr typed the blackmail letters and produced address labels himself, as well as mixing the infant formula with the 1080 and splitting it between the two envelopes while wearing gloves.

He drove to Marton one night and posted the packages from Paraparaumu the following day.

A criminal investigation started when they were received by Fonterra and Federated Farmers in November 2014 threatening to poison infant formula if New Zealand did not stop using 1080 by the end of March.

The public was told of the threat in March and formula was taken off supermarket shelves and held securely to prevent contamination.

More than 2600 people were considered over the course of the investigation dubbed Operation Concord, which cost police $3 million, Police Commissioner Mike Bush revealed when the man was arrested in October.

Blackmail is punishable by up to 14 years in jail.