A video has emerged showing unwanted calves being violently abused.

Animal welfare organisations SAFE and Farmwatch started an investigation into the dairy industry in August.

They set up hidden cameras across 12 farms and a slaughter house, which shows animal cruelty, including a man violently kicking and throwing helpless calves before bashing them and slitting their throats.

SAFE executive director Hans Kriek said each season more than 2 million unwanted calves, referred to as bobby calves, were killed as young as four days old.

"In order for a cow to produce milk, she first has to produce a calf, and these calves are often surplus to requirement by the industry so they are removed from the mothers quickly after birth and many of these calves are then taken to the slaughter house."

"They're being kicked, they're being thrown and the footage that people are upset about was only a fraction and it didn't show the worst."

Mr Kriek said he had made a complaint to the Ministry for Primary Industries but had not received a response.

Warning: Video contains images of animal cruelty.

Federated Farmers dairy section chair Andrew Hoggard said it was appalling behaviour from a minority of people who worked in the industry and would not be tolerated.

He said farmers had to work within strict animal welfare rules and the vast majority cared for their stock humanely.

DairyNZ said it was taking the issue very seriously and was working closely with other industry partners. The Ministry for Primary Industries has not been available for comment.