The letters were "extremely nasty" and were referred straight to the police.
Recently there had been a significant increase in threats and abuse against DOC staff, both in person and online, he said.
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," Sanson said.
"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."
Central District Crime Manager Detective Inspector Ross McKay said DOC staff should be able to go about their lawful business without the fear of threats and intimidation.
"While we understand the strength of feeling by some in our community about the use of 1080, it is never acceptable for this to be expressed in the form of threats to individuals," he said.
"We take very seriously any threats of this nature.
"While I am unable to go into details of our investigation at this stage as matters are now before the court, I can say that it has been tenacious and extensive.
"Anyone considering resorting to unlawful means to advance their cause in this way should think very carefully about the consequences of their actions, and know that police will thoroughly investigate."
The man will appear in New Plymouth District Court tomorrow.
Police were also continuing to investigate the illegal release of Sika deer into the Taranaki conservation area, McKay said.
