Gregory Ross Buchanan has pleaded guilty to a charge of blackmail after making threats against the Department of Conservation in letters. (file photo)

A Taranaki man has pleaded guilty to a charge of blackmail after sending threatening letters to the Department of Conservation.

Gregory Ross Buchanan appeared before Judge Garry Barkle in the New Plymouth District Court on Thursday facing four charges of blackmail.

The 65-year-old had denied the charges, which relate to a series of threatening letters he sent to DOC over its use of the controversial pesticide 1080, between September 25, 2017 and November 6, 2018.

Defence lawyer Patrick Mooney entered a guilty plea to one charge of blackmail after three others were withdrawn.

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Mooney said there was still an an issue with the summary of facts with the Crown claiming rat poison had been in one of the envelopes posted to the department.

"The Crown have had analysed a substance that was found in an envelope and say it is rat poision."

He said Buchanan claimed the substance was in fact chalk.

In the letters Buchanan threatened to release Sika deer into the DOC estate, including Egmont National Park, and to poison meat and milk processing plants, in an effort to force the department to stop using 1080.

The pesticide has been regularly used in the Egmont National Park to control pests, including possums, rats and stoats, which are blamed for damaging bird life in New Zealand.

Opponents claim the poison effects water and other animals, but DOC scientists have refuted the claims.

Buchanan was arrested on November 6, following an extensive police investigation.

He was remanded on bail, which includes strict conditions not to enter New Plymouth's DOC office, not to have contact with any DOC employees and not to enter onto any DOC land, for sentencing.

At the time of Buchanan's arrest DoC director-general Lou Sanson said there had been a significant increase in threats and abuse against DOC staff centred around its aerial use of 1080 but it was justified in doing so.

"1080 is the best tool for the job in large, rugged or remote areas and if we don't use it forests will continue to be devastated by pests and we could lose our precious birds such as kiwi, mohua, and kokako.

"Where we are using 1080 to target predators, our native species have started to recover."

Sanson thanked police and the department's staff who had worked on the investigation.

"It is completely unacceptable for anyone to threaten or intimidate DOC staff.

"This case demonstrates that action will be taken when any criminal offending occurs."