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The department received four letters of a threatening nature relating to the use of 1080 between September 25, 2017 and November 6, 2018.
Gregory Ross Buchanan, 65, was arrested last year following what police described as an extensive investigation.
After entering deemed not guilty pleas in November, Buchanan's lawyer Patrick Mooney today told New Plymouth District Court his client would plead guilty to one charge of blackmail, with the three others that had been laid against him to be withdrawn.
Mooney told Judge Garry Barkle there remained "an issue" in the summary of facts agreed between the Crown and the defence.
The Crown asserted that one of the posted envelopes had contained rat poison, but Mooney told Barkle the substance was actually chalk and had been analysed.
Mooney said the matter would need to be resolved prior to Buchanan's next appearance
for sentencing, on May 8.
He was remanded on bail, under conditions that prohibited him from entering New Plymouth's DoC office or approach DoC staff.
DoC has reported a significant rise in threats and abuse against DOC staff, both in person and online.
Threats were mainly centred around the department's use of aerial 1080 to rid conservation land of rats, stoats and possums which are decimating New Zealand's native bird species.
"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," DoC director-general Lou Sanson said at the time of Buchanan's arrest last year.
"Where we are using 1080 to target predators, our native species have started to recover.
"DOC staff need to be able to get on with their job of protecting conservation areas without fear of being harmed or harassed."