The Department of Conservation has initiated an independent review of a 1080 drop in which cows were killed.

DoC says it is taking the "extraordinary step" to provide public confidence in its operations.

Eight cows were found dead on a King Country farm following DoC's Mapara 1080 helicopter drop on private and conservation land on September 6.

Toxicology results on two cows confirmed they had ingested fluoroacetate (1080), after they broke through a fence to get into the operational area.

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DoC said initial investigations of the site, permissions and communications documents and helicopter flight data indicated DoC had undertaken the operation as planned and cattle did not have access to 1080 bait on the farmers' paddocks.

A pre-flight of the operational boundary was conducted a fortnight before and staff noted stock in the neighbouring bush block and advised the farmers, Paula and Mark Stone, to remove the stock.

The farmers confirmed these stock had been removed, however a post-operational inspection showed a gate had been pushed over, DoC said.

Paula and Mark Stone told RNZ communication about the operation had been poor.

They said 1080 was dropped next to the area they were told to move the cattle to rather than the bush next to the area they were moved from.

They said it was possible the animals escaped into the operation area.

DoC said they had obtained written permission from the landowner for the operation to include part of their land, and provided them information packs during the consultation period.

They also got written assurance from the land owner that they understood the details of the operation and areas concerned. During operations landowners guarantee their stock will be behind secure fences.

DoC's acting director general Mervyn English said the review was important to ensure the public could have confidence in the professional discipline DoC brought to its operations.

Waikato Regional Council will undertake the independent review.