Eight cattle have been found dead on a King Country farm after a 1080 drop in the area , prompting a Department of Conservation investigation.
DOC said it appeared the cattle had breached a fence line into the pest control operational area and the department was working with the landowner to work out how it happened.
Samples of the dead animals were taken for analysis and the department expected results back within the next fortnight.
The pest control operation in Mapara took place on September 6 to target rats, stoats and possums using aerially applied 1080 baits.
The operation included comprehensive consultation with adjacent land owners.
DOC said staff were alerted to the dead cattle on a farm adjacent to the operation area. They visited the farmer, accompanied by a vet, assessed the area and took samples from the dead animals.
The site inspection indicated cattle had entered into the operational area through a broken fence line. During a pre-flight of the operational boundary a fortnight before the drop, DOC staff noted stock in the operational area and advised the farmer to remove the animals.
Landowners Paula and Mark Stone told RNZ the 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 Operations Director David Speirs told RNZ the evidence pointed to the animals escaping on to a paddock in the drop zone.
He said no bait was found in the area where the cows were found dead.
Speirs said a review of the operational data showed the aerial drop went according to plan and as agreed with all adjacent landowners.
The helicopter GPS flight lines showed no over-flight of the adjacent farm area and there was a 50m buffer within the operational area in place, he said.
"We have been working closely with the landowner concerned to confirm exactly what happened, and also to support them as any good neighbour would under these circumstances with the burial of the dead animals and we have offered to assist with feed for the remaining animals," he said
DOC said nearly 30 years of pest control in the area had resulted in one of the most important strongholds for kōkako on mainland New Zealand and this area was a key source site for establishing new kōkako populations.
New Zealand falcons and North Island robins are also found in the reserve.