An investigation into a photograph of thin cattle being transported on a Cook Strait ferry has revealed no evidence of physical harm.

The Ministry for Primary Industries (MPI) has completed an investigation into an animal welfare complaint regarding jersey cows being transported across Cook Strait in March this year.

The cull dairy cows were being transported from a farm in the upper South Island to a meat processing plant in Wellington.

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MPI Nelson Marlborough district compliance manager Ian Bright said they spoke to a number of people in relation to the complaint and found there was no evidence of an animal welfare offence.

"After speaking to all people who handled the animals from the farm to processing, there was no evidence of the animals suffering physical harm through poor body condition.

"That is not to say they were in prime condition. These were dairy cows that were being culled from the herd and being sent for processing," Bright said.

"Their body condition was acceptable for transporting and for processing."

Inspectors interviewed the farmer, the stock agent who arranged the transport, the truck driver, the transport company owner, the yardman at the processing plant and the veterinarian at the plant who inspected the animals on arrival, he said.

Staff also visited the farm where the cows came from and found no animal welfare issues, Bright said.

Staff spoke to the farmer's vet and stock agent who both expressed confidence in the practice on the farm and the reputation of the farmer, he said.

"We found that the 50 cows were inspected by a veterinarian when they arrived at the processing plant, who found no issues with poor body condition."

MPI receives about 700 animal welfare complaints a year in relation to farm animals. Most of those complaints related to stock in poor condition.