The man who threatened to contaminate infant milk formula with 1080 said he "just snapped" and immediately regretted his actions.

Jeremy Hamish Kerr, 60, of Mellons Bay, East Auckland, pleaded guilty in December to two counts of blackmail over the fiasco but did not accept the assertion his actions were motivated by money.

A disputed facts hearing this week, designed to give Justice Geoffrey Venning greater clarity around Kerr's motives, has been adjourned until next month for further evidence to be produced.

A police interview from October was played in the High Court at Auckland today, which revealed how authorities were led to the defendant.


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

"I find that absolutely staggering. Amazing," the defendant said.

But as police piled up the evidence he came clean about being behind the plot. Packages were sent to Federated Farmers and Fonterra in November 2014 threatened to poison infant formula if New Zealand did not stop using 1080 by the end of March.

"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, instead claiming he was triggered by a media story he saw about 1080 and comments by Nick Smith.

"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."

Kerr also mentioned he was grieving after the loss of his wife a few years earlier but was unable to respond to many of the police's questions about what drove him to such extreme measures.

"It was dumb. The whole thing was dumb. I just wanted it to go away," he said.

"I can't describe what I was thinking. I was pretty stressed."

A search of Kerr's property turned up a laptop on which a "retraction letter" was written. Police were also led to the 60-year-old after testing the 1080 he sent.

They tested the sample against 56 others from around the country and were eventually able to whittle it down to three; two of which came back to Kerr and one to a company with which he had links.

At the end of the October interview, Mr Pascoe arrested Kerr, read him his rights and asked if he had anything more to say.

"Sorry," he said.

The Crown says Kerr, as the creator of a pest-control product and 1080 alternative Feratox, had a financial motive for the crime.

Christine Gordon, QC, said had 1080 been banned, Feratox would have been the "natural replacement" and its use would have spiked to control the possum population.

The defendant earned 10 per cent in royalties from each sale, so would be in line for a large windfall.

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

But Kerr told police the royalties totalled $100,000 a year and he was "not in any stress" over his bank balance.

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

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

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.

Kerr will be sentenced next month and could face 14 years behind bars.