The farm monitoring group that took hidden camera footage showing a Northland contract milker hitting cows with a steel pipe says MPI is "incompetent when it comes to animal welfare issues".
Farmwatch hid cameras in the shed earlier this year.
It alleges over a month-long period it recorded a male contract milker routinely hitting and beating cows, at times with a metal pipe.
The group has not named the farm or the contract milker, but said the footage has been handed over to the Ministry of Primary Industries (MPI).
MPI is now investigating the farm.
Watch John Campbell's interview with John Darroch here (warning, contains images of animal abuse):
Farmwatch spokesperson John Darroch said MPI was only acting because the animal rights group had gone public with the footage.
"The footage is horrible, it shows this contract milker rushing at the cows, screaming at them, yelling at them, swearing at them and beating them with a range of different objects.
"The cows in the footage are terrified, they're trying to back away from him, they're trying to get away from this guy and they're all terrified of going into the shed. They don't want to go in and get milked because they know that they're going to get beaten."
Mr Darroch said Farmwatch only put hidden cameras in the milking shed because a farm worker, who had witnessed the abuse and complained about it to MPI, was told they could not prosecute without video footage and the case was being closed.
"They came back and told the whistle blower ... 'the farmer is a nice guy'."
Mr Darroch said two people went to MPI, including an ex-worker. He said MPI went to the farm and spent about half an hour there before the cameras were installed.
"They came back and told the whistle-blower that quote 'the farmer is a nice guy'.
"They had specific knowledge of exactly what was going on, and they closed the investigation despite that.
"In my opinion MPI has proven incompetent when it comes to animal welfare issues. You can't trust when you go to MPI and tell them about animal abuse that they'll be able to uncover it and prosecute based on it."
He said MPI needed to assure whistleblowers it would take complaints seriously, but the Ministry should be stripped of animal welfare responsibilities.
"They've shown time and time again, year after year that they're incapable of uncovering quite obvious and systematic abuse."
In a statement MPI said it began an immediate investigation into the footage as soon as it received it from Farmwatch last week.
It did not respond to questions about whether the contract milker had been subject to animal abuse complaints in the past.
"As this is an active investigation, we cannot comment further at this stage," a ministry spokesperson said.
MPI also refused to be interviewed by Checkpoint.
'If a farmer treats their cows badly, they shouldn't be working in dairy' - Dairy NZ
Dairy NZ - which represents dairy farmers - said the footage was disturbing, but this behaviour was not representative of the thousands of farmers who worked with cows every day, and were passionate about animal welfare.
"Cruel and illegal practices are not in any way condoned or accepted by the dairy sector as part of dairy farming. If a farmer treats their cows badly, they shouldn't be working in the dairy sector. It's as simple as that," Dairy NZ said.
A Northland sharemilkers' spokesperson said he was horrified to see the abuse inflicted on cows by the contract milker.
Northland's sharemilker rep on Federated Farmers Damian Dixon said the footage was sickening and hard to watch.
"It's pretty horrific treatment of animals," he said.
"I just have a feeling there might be some underlying issues with that man and hopefully the Rural Support Trust will get involved and talk to him and see what's going on."
However, Mr Dixon said the abuse displayed in the video could never be condoned and in 19 years on dairy farms, he had never encountered such behaviour.