Bay Bush Action posted a photo of dozens of kiwi corpses collected at Opua State Forest on its Facebook page.
It said nearly all the birds were killed by dogs who were not kept on a leash.
"Even the most harmless beloved dog can be a killer of a kiwi: They need to stay on a leash where kiwis might be,'' the organisation's statement said.
Northland has the highest population of kiwi in the country.
Despite that, the average lifespan of kiwi in the area is 25 years, compared to about 60 years in other parts of New Zealand.
In 2008, the population of Northland brown kiwi was calculated at around 8000 birds, living in 25 clusters from Tawharanui to Whakaangi.
It is estimated that the number of North Island kiwi declined by at least 90 per cent during the 20th century.
