Initially, police said the deaths on Peter and Janine Stackhouse's farm, along with another 20 on the neighbouring Tapui property of John and Wendy Dodd, could have been caused by gunshots.
However, yesterday Oamaru police said intensive tests had found the sheep were attacked by dogs, but there was insufficient evidence to prosecute anyone.
Otago coastal area commander Inspector Jason Guthrie said dogs were located in the area around the farms at the time of the incidents, but there was insufficient evidence to indicate they were involved.
Mr Stackhouse said yesterday he initially suspected dogs had killed the sheep.
''I had my doubts at the time but, even now, as far as I'm concerned nothing has been proven. Where are they [the dogs], will they come back, and whose are they?'' Mr Dodd did not want to comment.
Both wanted to put the incidents behind them and move on.
The sheep were killed on the two properties over two separate weekends, the second incident causing fear among the community that culprits with rifles would return again.
The initial investigation indicated the sheep had been shot but there were several inconsistencies.
No bullets or casings were found at the sites.
Further investigation included firearm testing, a forensic veterinarian and forensic testing in Australia.
It concluded the sheep were killed by dogs.
Police have established no firearms or humans were involved in the deaths.
Insp Guthrie said the investigation was unusual because of the numbers of sheep attacked and the type of injuries they had.
''It was a difficult case to unravel, because of the severe and unusual injuries that were sustained by most of the sheep during the attacks.''
The killings caused a high level of anxiety in the Ngapara community and surrounding areas.
As a result, police conducted patrols in the area to reassure them.
''I hope that the police findings will alleviate any remaining fears among the local community,'' he said.
Police thanked people for all the information provided both locally and from around the country, and for the support shown to the two affected families.
