Warning: This story contains distressing images of dead kiwi.
A Bay of Islands conservation group is pleading with dog owners to keep their pets under control after the death of at least six kiwi in the past fortnight.
All six birds were found at the Paihia end of Ōpua Forest, where Bay Bush Action has been working since 2011 to eradicate pests and bring back native wildlife.
Volunteer trappers found the two most recent kiwi corpses on Sunday morning.
All were discovered within a roughly 500m radius and had injuries consistent with being gripped in a dog's jaws.
Three of the kiwi, including the latest two, have had DNA swabs taken and been sent away for post-mortem examinations.
Bay Bush Action co-founder Brad Windust said the deaths were devastating for the volunteers, who had spent countless hours working to build up kiwi numbers.
"It rips my heart out," he said.
"They're like our children, and we feel such a responsibility to look after them."
One of the dead kiwi was the biggest breeding female he had ever seen. He had called it Tuakana (older sibling) because it was likely the population founder.
Tuakana had survived the twin threats of stoats and cats as a chick, only to be killed by an out-of-control dog, Windust said.
No kiwi were heard in the area when Bay Bush Action started 12 years ago.
Hearing the first kiwi call several years ago sparked great excitement among the volunteers, and numbers had slowly increased since then.
At last count, before the spate of killings, there were an estimated 22 kiwi in the area.
Now the bush had fallen silent again.
"It's terrifying to think how quickly that painstaking work can be undone through careless dog ownership," Windust said.
The group was working with the Department of Conservation in a race to find the dog, or dogs, before more kiwi were killed.
Windust feared the dead kiwi found so far were just the tip of the iceberg.
"We are probably only finding a tenth or less of the actual kiwi that have been killed. Just a few weeks ago, our valley was alive with the sound of kiwi calling, and now there's silence. It's heartbreaking."
Fellow trustee Catherine Langford said the first dead kiwi, a juvenile male, was found by a hiker on Oromahoe Traverse Track on 23 July.
Two more were found on 31 July, but appeared to have been killed about the same time as the first. The fourth was found on 3 August.
All were more than a year old and weighed more than a kilogram, so were capable of defending themselves against cats and stoats.
The fact kiwi were reaching that size was proof the group's efforts to eliminate pests were working, she said.
The volunteers were delighted when kiwi calls increased sharply last year.
"We were absolutely stoked to have reached that point, and it's devastating to be taking a backwards step now.
"It's heart-wrenching for the volunteers and for the trappers. They put so much effort into this work, so much heart and soul … To see it undone is quite traumatising for them."
It was also "a horrendous waste of time" for the volunteers after they had achieved so much, Langford said.
"Just one roaming dog can do so much damage," she said.
Windust said kiwi should live up to 50 years, but in Northland they survived on average just 14 years.
Roaming dogs, as well as dogs used by "unprofessional" pig hunters, were a major factor in the short lifespan of Northland kiwi.
A dog that had roamed once was likely to do so again, Windust said.
Owners needed to consider re-homing such dogs well away from kiwi areas, and all dogs needed to be tied up or contained when not being watched.
Geofencing, which created an invisible boundary linked to a shock collar, was a reasonably affordable alternative to traditional fences, Windust said.
He also urged pig hunters to run only a small pack with three, at most four, well-trained dogs.
They should stay right beside their owner unless they smelled a pig, and should always be fitted with tracking collars.
"Have target-specific dogs - if they're killing possums, they'll kill kiwi too. Just because they don't kill chooks at home doesn't mean they won't kill kiwi in the bush. And don't hunt at night in kiwi areas," he said.
Pet owners could take one of the free 'Know Your Dog' courses run by Kiwi Coast.