A complaint has also been laid with police in relation to 1080 baits being placed in a DOC worker's letterbox.
The poison 1080 is widely used by DOC to control rats, possums and stoats but some activists oppose it because they believe it harms native species.
DOC staff have previously been threatened and abused by anti-1080 protestors.
State Services Commissioner condemned the alleged tampering, saying it had put DOC staff and their families in danger of a serious crash.
"And putting poison in mail boxes of private houses is clearly an attempt to intimidate," he said.
"This unacceptable behaviour is coming from a very small group of people and is rightly under police investigation.
"Public servants deliver essential services and support to New Zealanders every day and they deserve to work in a safe environment where threats, abuse and violence are not tolerated."