Anti-1080 protesters have "declared war" on the Department of Conservation over its use of the pesticide. (file photo)

Anti-1080 protesters are vowing 'all out war' against the Department of Conservation with threats to take down its helicopters as they drop the controversial pesticide into Egmont National Park.

In a letter sent to the Department of Conservation and Fairfax Media the group, calling themselves the New Zealand Hunters, threatened to mount a war against the use of the pesticide.

"We are going to bring you bastard to your knees," it reads. "This will be a war like no other, you watch this happen around you. We the hunters of New Zealand have had enough of your bloody lies."

ANDY JACKSON/STUFF The letter threatened the "famous Egmont bush" on Mt Taranaki. (file photo)

DOC and the police have confirmed receipt of the new threats and were investigating.

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In October the group released at least 30 sika deer around the region and now claims to have released another 25 in areas from Waverley to Waitara. This move was described as "eco-terrorism at its worst" by the Department of Conservation's director-general Lou Sanson.

Sanson would not speculate on who wrote the letter or why, as it is now a matter for the police to investigate.

"However these threats to DOC staff are taken very seriously and will not be tolerated.'

"I am appalled that someone would threaten our staff in this way as they are trying to go about their daily work to protect our native species and wildlife."

Sanson said it was important staff were able to get on with their job of protecting conservation areas without fear of being harmed or harassed.

DOC has put measures in place to protect staff and contractors, including asking staff to be more vigilant as they go about their daily work.

Most New Zealanders supported DOC's work to protect native species and habitats and understood why it needed to use 1080 as part of the war against predators, he said.

Alan Simmons, co-leader of the NZ Outdoors Party, which has 8000 members, said he didn't believe genuine hunters would be involved.

"There is some absolute radical people and there's no telling what they would do but they are individuals," Simmons said.

"I can't believe that any organisation or group would have written that letter. No responsible people are going to come up with that sort of stuff."

The group claims it's going to target the "famous Egmont bush" and threatens to attempt to bring the country's meat and milk industries to its knees.

"Now listen, we will take down helicopters who support 1080 drops. We will take down people one by one, blood will fall. Watch this happen."

The letter says the group intend to pick up pellets and target open milk vats and the inside of meat processing companies, "with the bros on the inside".

"Be warned DOC these hunters have more guns than you or the police.

"There is no respect for you bastards any more. Let the war begin. Just be very careful, stop using 1080."

Detective Senior Sergeant Brent Matuku said the region's CIB, who were investigating the initial release of the deer in October, were aware of the fresh threats and were making inquiries.

Last month DOC operations director David Speirs said they were looking into how the deer were released, where they came from and whether there was a link with anti-1080 protests.

DOC hunters had killed eight of the deer so far, which all had holes in their ears consistent with ear tag identification, commonly used in farmed deer, and more had been sighted, he said.