Mice running through cages and the decomposing bodies of dead chickens floating in dirty water have been caught on camera by animal welfare activists at a Whangarei egg farm.
Direct Animal Action (DAA) activists shot the footage earlier this month at Zeagold Foods, formerly known as Forest Hill Farm, owned by Mainland Poultry.
The company said the conditions revealed in the footage were "completely unacceptable" and it was "extremely concerned" about the situation.
DAA member Dee Sims said the conditions the laying hens were living in were amongst the worst she had ever seen.
"What we found was pretty unhygienic conditions.
"We saw mice running through cages that contained hens, we found a cesspit inside the farm building where we saw a number of dead hens floating what looked like a combination of manure and water."
Many of the chickens had feather loss, raw red skin and looked generally unhealthy, she said.
Sims said she considered the living conditions "unacceptable" for the caged hens inside.
DAA had chosen to investigate Mainland's Whangarei chicken farm because the company was in the process of applying for a permit to build an industrial egg laying farm in the Waikato.
"We wanted to have a look behind closed doors to see how they look after their animals."
Mainland's land use change consent form revealed the farm would house about 600,000 laying hens and a further 200,000 rearing birds in 17 chicken sheds.
Each shed would contain up to 50,000 birds, most of whom would live in colony cages.
Sims said if this was how Mainland ran its farms, they should not be allowed consent to build a new one.
A Zeagold Foods staffer who gave his name as James said someone from Mainland was at the farm on Friday and they had discussed the video.
However he would not comment further about it.
"I've seen the video, and I can't really say anything," he said.
"It's just my bosses - they're dealing with it."
In a statement, a spokesman for Mainland poultry said he was "extremely concerned" about the conditions revealed in the footage.
"This is completely unacceptable and is not in any way reflective of Mainland Poultry farms, our animal welfare practices or operational standards."
He added that while the footage had been obtained illegally, in the dead of night and without permission, Mainland was nevertheless grateful the situation had been brought to their attention.
"A senior livestock management team will be on site [on Friday] to review and conduct an investigation as to why this farm was not following our approved procedures, operational standards and our rigorous standards for animal welfare."
The team would be implementing "immediate corrective action", he said.
"We are unable to comment further until we have conducted a full and thorough investigation, which we undertake to complete by early next week."
Mainland Poultry produces one-third of New Zealand's eggs, through its Zeagold Foods unit, including the Woodland and Farmer Brown caged and free-range egg brands.
In April this year the Herald reported Mainland needed to replace equipment to meet the requirements of the Animal Welfare (Layer Hens) Code of Welfare 2012, which calls for the staged phasing out of battery cages in favour of larger colony cage systems or free-range.
In March this year Countdown supermarkets announced they would sell 100 per cent cage free eggs by 2025.
They were followed by New Zealand's other supermarket giant Foodstuffs, which owns New World and Pak'nSave, which pledged to go cage-free by 2027 in September.