Greenpeace has shut down part of Whanganui's Whiskas factory after confirmation its tuna source is connected to slavery.
The operation is the heart of the cat food giants Australasian operations; the factory churns out over half a million pouches of pet food a day.
Greenpeace protesters, who travelled to Whanganui from Auckland, are livestreaming their actions on Facebook.
The protest was sparked after parent company Mars confirmed to Greenpeace that it sources tuna from Thai Union, a seafood company that has been connected to slavery and destructive fishing methods.
The environmental organisation claims six activists have been chained to a Greenpeace truck, branded with the Thai Union logo, since 8.30am, but only one can be seen on their live feed video on Facebook.
Activists are hanging a 14-metre banner reading "Stop Bad Tuna" on the side of the heavily branded building and have turned the company's two-metre high Whiskas cats into "cativists", giving them protest placards.
An activist locked in the truck tweeted: "We're shutting down the @MarsGlobal petfood plant in Whanganui because the tuna in Whiskas petfood is #NotJustTuna, it's #BadTuna."
Greenpeace campaigner Kate Simcock says Whanganui Police have visited the site.
She had a conversation with them, and they also went inside the Mars Petcare factory. They seemed pretty relaxed, she said, and respected the group's right to peacefully protest.
"I think they're playing the wait-out game at the moment."
The plant manager for Mars Petcare in Whanganui, Derek Pickering, said the company is aware of changes needed to the Thai fishing industry and was working with a third party to make improvements in that country.
He said the company had also been talking to Greenpeace prior to today's actions and didn't understand the protest.
Mr Pickering said Mars Petcare "does not tolerate forced labour in any aspect of our supply chain and we are extremely concerned about allegations of abuses taking place in the Thai fishing industry.
"We believe that as a global business we have a responsibility to contribute to the change that the Thai fishing industry needs to make to become a reliable and sustainable supply chain. As such, it is vital that we engage with, rather than abandon the industry, to improve conditions and to stamp out any human rights abuses.
"We do not believe that targeting one supplier is driving the change needed in the region.
"We are in conversation with Greenpeace and they are aware of our plans and therefore don't understand their actions taken today."
Mr Pickering said the Mars Petcare and the "reputable third party organisation" would work together "to establish transparency and visibility across all the tiers of our supply chain within Thailand".
Those findings will be ready in July and the report would be made public.
In July last year, the New York Times broke a story connecting the use of forced labour to the Thai Union Group, who supply seafood to pet manufacturers and tuna companies around the globe.
The Whanganui factory uses Thai Union sourced tuna in Whiskas' pouches that are sold in New Zealand and Australia.
"Mars have known about human rights abuse in their seafood supply chain for at least ten months," says Ms Simcock.
"Given this, it's shocking that Mars customers could still be buying Whiskas pouches containing seafood that may have been caught by slave labour using destructive fishing methods.
"It's time Mars publicly acknowledge these serious problems in their supply chain and tell us what they're going to do to ensure they're not selling products contaminated by modern-day slavery, overfishing and the needless slaughter of endangered sharks and other animals. Workers and our oceans deserve immediate action."
Globally, Greenpeace has been campaigning on Thai Union, since October last year.
As New Zealand activists are stopping the company's bad tuna from being packaged into cat food, the crew of Greenpeace's largest and fastest ship, the Esperanza, is pulling up the destructive fishing gear that catches Thai Union's fish.
"Exploiting people and our oceans seems to go hand-in-hand," says MS Simcock.
"The oceans don't belong to industrial fishing companies, they belong to all of us. Thai Union is the world's biggest exporter of canned tuna, they have products everywhere. Consumers, retailers, and customers expect them to fish sustainably and respect human rights."
The idea that cats may be eating food connected to serious human rights abuse, overfishing, and destructive fishing methods that kill sharks, rays and turtles will be "abhorrent" to cat owners everywhere, she says.
Ms Simcock said the activists would remain in place "until we're really sure Mars has got the message".