Rangers stopped counting the number of totara trees they found dead in the Raukumara Forest Park after they reached 200.
The Department of Conservation has been undertaking survey work in the area for a number of years and has linked changes in the forest to introduced deer and possums.
In November, a survey of the understory in the forest was done, with DOC finding 200 dead totara.
Its east coast biodiversity ranger, Graeme Atkins, said staff stopped counting because it became too depressing.
DOC spends over $250,000 a year controlling red deer and goats within the forest, but there is no possum control undertaken at the moment.
But the forest park is open to anyone wishing to trap possums, who eat the leaves and fruits of the totara, for the commercial recovery of fur and skins.
The last large-scale possum control operation in the area was 20 years ago and involved aerial application of 1080 pesticide.
"(The number of trees dying) indicates that a high portion of totara are being impacted on by possums and the bigger picture is a combination of feral goats, feral deer, rats, and so on, are having an ongoing impact on the forest and wider economy," DOC operations manager John Lucas said.
"We know that where we undertake pest control we get positive results.
"Different types of pest control are suited to different areas and we need to do some further work and consultation to determine what's the best action for the Raukumara Forest Park."