Department of Conservation staff working on pest control have had the wheel nuts on their cars loosened, apparently by anti-1080 protestors.

The department has laid a complaint with the police after three cars being driven by a DOC staff member, a contractor and a member of the public were allegedly tampered with.

In the worst case, the wheel of the DOC contractor's car came off while he was driving and jammed in the housing around the wheel.

DOC director-general Lou Sanson said it was fortunate that no one was hurt. His department believed the alleged tampering was designed to intimidate staff working on aerial pest control operations involving the poison 1080 - though not all of the cars affected were working on pest control projects.

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"People have a right to lawful protest, but I will not tolerate anyone putting my staff at risk as they go about their job of protecting our precious wildlife," he said.

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."