Few signs were evident of Saturday night's blaze, which killed 400 pigs at a farming operation in Hopuhopu.

About 400 mother and baby pigs were burned to death after fire ripped through the shed they were being housed in, on a farm north of Ngaruawahia.

It is a case of history repeating at the Hopuhopu piggery, where 300 of the animals were burned to death in a similar blaze in April 2005.

People driving past the farm, which is not far from State Highway 1, raised the alarm around 8.20pm on Saturday night and firefighters were quickly on the scene - but too late to save the occupants of the 800sq m "farrow house".

File A photograph taken in 2005, at the piggery, where a similar fire killed 300 pigs.

The sows and piglets would have been housed in metal pens and had no chance of escape once the fire took hold, quickly becoming an inferno due to the flammable polystyrene cladding in the building's walls.

Firefighters spent about five hours quelling the blaze.

Brien Farms owners Ray and Joanne Brien have owned the business on Great South Road for about eight weeks.

"It looks like a bit of a war zone down there," said Ray Brien of the remains of the shed, now a twisted pile of iron sheeting.

"It is pretty bloody awful, but it could have been worse. The guys from the Ngaruawahia and Huntly fire brigades did a great job to stop it spreading. There were 500 pigs in the building right next to it, and they saved that one."

The couple were still traumatised by the blaze on Sunday afternoon and did not want Stuff to go too near the incinerated shed and the bodies of the animals.

"I was asleep at the time. The first I knew of it was when one of the guys that works for me was banging on the door," Ray Brien said.

"It was great how everyone came from around to help. The farmer next door came over with his digger, and used it to pull down the side of the shed so the firefighters could put the fire out."

A Maori Warden living nearby even took up station on the farm gate, to ensure only those who needed to get to the fire site were able to get through.

Fire investigators were yet to pick through the site, but the couple suspected an electrical fault may have been the cause of the blaze, even though the building was about five years old.

In total, the couple farm about 3500 pigs on the property, which is fully insured. They sell their stock direct to Hellaby Meats.

Joanne Brien said: "We really want to thank the fire brigades who were just amazing. We are just thinking about what we can do to show our thanks. And we really want to thank whoever was driving by and saw it and called the emergency services.

"We really care about our animals' welfare. It really hurts that this has happened to them."

The Briens said they had no cause to suspect foul play.

"There are a lot of activists and that sort of thing, but no one would want to do that to pigs."

They had taken up pig farming after selling a dairy farm they owned near Otorohanga.

"We left there for a better life, but since we have been here we have had nothing but trouble with equipment failures and all sorts of things," Joanne Brien said.

"We were just getting on top of everything and then this happens. It's a real kick in the guts."

The last big fire at the farm in April 2005 killed 300 pigs. Amber Johnston, who helped alert nearby people and Joanne Brien, who was out of the area at the time, remembered that event, which she said was caused by welding equipment.

Ngaruawahia chief fire officer Karl Lapwood said as soon as the brigade arrived it was obvious the building on fire could not be saved.

"All we could do was save what we could save.

"We got a low-pressure water spray between that [the building on fire] and the building next door, which was filling with smoke.

"The guys went in with breathing apparatus and broke out all the glass windows, and we got some portable fans in there to pressurise the room."

Adding to the danger was a 3000-kilovolt power line directly overhead.

"It made things pretty tricky. It was right up there in terms of the big jobs we have dealt with."

Hans Kriek, the executive director of animal welfare advocacy group Safe said the piggery was not on his organisation's radar, although it was distressing to hear that it was the second time a blaze there had killed hundreds of pigs.

"You would hope that if it has happened there before, they would have implemented some safeguards to stop this sort of tragedy from happening again. Quite often these sorts of farms can be quite run down and the owners have to do a lot of work to meet the safety requirements and the code of welfare."

Kriek said he hoped that the fire would serve to highlight the benefits of free range pig farming, rather than the more intensive and less humane options.

"In those sort of facilities if something goes wrong then there is no escape for these animals.

"Accidents can happen and I would not want to say anything bad about these people without knowing anything about them ... [but] some of these operations are pretty terrible.

"You go there and you see hundreds and hundreds of rats running around. When you have conditions like that, it is just an accident waiting to happen."

Representatives of industry body New Zealand Pork were not able to be contacted.