The Department of Conservation has laid complaints with police over alleged incidents of its vehicles being tampered with. (File photo)

The Department of Conservation (DOC) says its vehicles have been tampered with in what's thought to be an effort to intimidate staff and contractors and disrupt 1080 pest control.

In the worst case, a contractor's wheel came off while he was driving, and jammed in the wheel housing. The wheel nuts had allegedly been loosened. A spokeswoman said it was lucky the contractor had been driving at about 5kmh at the time.

DOC has laid complaints with police over the incidents, which are said to have occurred over the past three weeks.

SUPPLIED Two of the instances of tampering with vehicles had occurred in the North Island and one in the South Island, DOC said. (File photo)

There had been three cases where wheel nuts were allegedly loosened on DOC, private and contractor's vehicles in two different regions. Two had occurred in the North Island and one in the South Island, the spokeswoman said.

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DOC Director-General Lou Sanson said it was fortunate no one was hurt by potentially very dangerous events.

"I believe the activity is designed to intimidate DOC and OSPRI workers and disrupt aerial 1080 pest control operations, although not all vehicles targeted were being used to support operations," he said.

The department has also informed police of one case where 1080 baits were left in a letterbox at a DOC address.

"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," Sanson said.

"We take any threats to DOC staff very seriously and are working with the police to find those responsible."

DOC had put measures in place to protect staff and contractors and was being vigilant in checking all vehicles before use. It had asked contractors to do the same.

This year's programme of aerial 1080 and ground-based predator control - the Battle for our Birds programme - was well underway. It was "critical to DOC's goals of protecting threatened species and working towards making New Zealand predator free by 2050," DOC said.

OSPRI undertook aerial 1080 operations as part of its TBfree​ pest control programme, which also benefited wildlife, DOC said.