Shocking helicopter footage of Russell State Forest shows tōtara, northern rātā and pūriri dying on a large scale.
Forest & Bird has released the film days ahead of an aerial 1080 operation planned to turn around the fortune of the collapsing forest.
The proposed aerial operation follows years of pressure from Forest & Bird.
"The collapse of this forest has happened slowly, over decades, and mostly at night. Since possums arrived in Northland during the late 1950s and early 1960s they have eaten their way through every part of the north," said Forest & Bird's Northland Forests Advocate Dean Baigent-Mercer.
"Armies of possums and rats are munching through leaves, flowers, fruit and seedlings every single night. These introduced animals, together with stoats, ferrets, weasels and feral cats, are progressively wiping out our wildlife. Native birds, bats, lizards and insects are disappearing."
"We need to target all pests for our native forests to thrive, and 1080 enables us to do that."
Between 1979-93 wildlife surveys showed an 80 per cent decline in kūkupa (kererū) numbers in Russell State Forest.
"People have forgotten what healthy native forests are like. We have inherited loss. But northern kaumātua in their 70s and 80s can remember flocks of 30-40 kūkupa as children. That was normal back then and we can have that again."
Mr Baigent-Mercer says it was crucial that fear and fake news recently stirred up by anti-1080 protests did not derail essential 1080 operations.
"If Forest & Bird – an independent organisation set up to protect our native forests and birds, running for nearly a hundred years – thought that there were mass killings of native birds from the way 1080 is used today, we would be the first to oppose it. But it's not happening. It's just the opposite.
"The evidence is very clear - endangered species are being pulled back from the brink of extinction, more common native birds are exploding in number, dying ancient native trees are coming back to life and there's carpets of seedlings that would otherwise be eaten by rats and possums.
"Aerial 1080 is an essential tool in the pest control toolbox if we are to restore our dying forests and wildlife. No other method of pest control can knock down the worst introduced predators over difficult terrain in three days and three nights," said Baigent-Mercer.
"We should not be letting native forests get as sick as Russell State Forest before we use 1080."