The man who exposed calves being cruelly mistreated on dairy farms said he was tipped off by concerned farmers and rural community members.
Farmwatch investigator John Darroch said he saw deliberately cruel treatment of calves at about 15 of the 50 farms he secretly filmed across the Waikato.
"We saw calves being torn from their mothers and left in the hot sun for hour after hour, thrown into trucks and then beaten to death," he said.
Secret video recordings also showed bobby calves being kicked and thrown about at a slaughterhouse.
Between eight and 10 transport companies were filmed transporting calves -- and all treated them in a cruel manner, Mr Darroch said.
"I wasn't expecting to see this -- not in the slightest.
"We have been getting calls from people in rural communities -- including farmers -- for many years asking us to look into the treatment of bobby calves.
"But I had no idea that every time we posted a hidden camera we would get brutal treatment of calves. The scale and the frequency absolutely stunned me."
The footage, filmed in 2014 and this year across farms and a slaughterhouse in the Waikato, was screened on TV One's Sunday programme last night.
Mr Darroch said he wanted to expose what went on in farms, rather than to hurt farmers.
He suspected the mishandling of bobby calves had been going on forever, as they were considered a byproduct of the dairy industry.
"Calves being separated from their mothers at a day old is integral to the dairy industry -- we don't have dairy products without that. Even on the best farms with very high welfare standards, they're still pulling their babies away from the mother at a day old."
Consumers needed to think about whether they could accept this when they used dairy products.
DairyNZ chief executive Tim Mackle said cruel and illegal practices were not condoned by the industry.
"We are shocked and farmers are too," he said.
"We will be asking questions of everyone involved. Farmers don't see what goes on when calves leave their farm and we need to be holding the transport operators and processing plants to account to ensure bad practices get stamped out of our industry."
Federated Farmers spokesman Andrew Hoggard said the appalling behaviour was from a minority of farmers, transport companies and slaughterhouse workers and it was not something the industry would tolerate.
Hans Kriek, executive director of animal rights organisation SAFE said the video clearly showed the animals being abused.
He said more than 2 million bobby calves were killed each season, four days after being born, and were "literally left like rubbish, to be picked up at the side of the road".
"The moment we saw the footage we realised there was a serious problem happening especially at the slaughterhouse," he said.
Mr Kriek said a complaint was made and footage provided to the Ministry for Primary Industries on September 14, but there had been no response.
"It appears that MPI didn't even bother to visit," Mr Kriek said. "They were given hours and hours of hidden camera footage of the abuse."
Matt Stone, from MPI, told Sunday the ministry was investigating.
"We have initiated an investigation and I'm not going to comment further on our processes around that investigation," he said.
The owner of the slaughterhouse concerned claimed it had not been approached by MPI.
Fonterra group director co-operative affairs Miles Hurrell described the footage as "disgusting".
Mr Hurrell believed, however, that only a "small minority" of dairy farmers in New Zealand were guilty of animal cruelty.
"The vast majority of our farmers operate responsibly, and this is really sad to see this footage come to light," Mr Hurrell said.
In a statement, Fonterra said it would take immediate steps to deal with any mistreatment of animals by its farmers.
Steps included meeting with SAFE to discuss concerns and seek further information from them about the footage. Fonterra also planned to get in touch with representatives from the meat industry and express concerns about the treatment of bobby calves.
"While bobby calves will always be part of the dairy industry, they must be treated with care and respect. Behaviour in this footage in no way represents the vast majority of New Zealand farmers who care about their animals," the statement said.
"As a co-operative we take a hard line on animal welfare. We're investigating this and will be taking strong action if any of our people were involved."
MPI deputy director-general regulation and assurance Scott Gallacher said an investigation by MPI began immediately after footage was provided to the ministry in September.
Mr Gallacher said the cruelty shown in the footage was unacceptable.
"Anybody seeing this type of abuse would be appalled."
He said MPI took animal welfare seriously and there were clear laws and guidelines for how animals should be treated.
The footage shown in the programme was part of extensive footage given to MPI by Farmwatch on September 14.
Animal welfare officers reviewed it and immediately began an investigation, he said.
"MPI must build a robust case before proceeding with any enforcement. MPI cannot comment on the specifics of the investigation as it may prejudice its outcome. We informed Farmwatch we would look into it."
The codes of welfare concerning bobby calves are clear on the following issues:
• It is not acceptable to throw, kick or hit animals
• Using blunt force to euthanise bobby calves is unlawful except in emergency situations
• Calves must be well fed, have comfortable accommodation and be fed within two hours of being transported
• People who transport calves need to make sure they don't cause any unreasonable pain or distress.
"MPI, in conjunction with industry, does a lot of work with farmers, transporters and processors to ensure the humane treatment of bobby calves and that everyone working with animals understands their obligations.
"MPI gets about 700 animal welfare complaints a year and follows them all up. Where there is offending, people are held to account.
"MPI has a strong record of successful animal welfare prosecutions and the penalties are significant."
The maximum penalty for wilful ill-treatment is five years in prison and $100,000 for individuals, and a maximum of $500,000 for a company.
A former dairy farmer was sentenced to four and half years in prison in October for wilful ill-treatment of animals.
An additional $10 million in new funding was provided in the 2015 Budget for animal welfare. Parliament has passed the Animal Welfare Amendment Act 2015 to improve compliance and enforcement.
"We strongly encourage all people who have information about the ill-treatment of animals to contAct MPI immediately on 0800 00 83 33."