Medical doctor Lisa Searle attached herself to an access gate at the state-owned corporation's base in West Pennant Hills before dawn on Tuesday and says she's staying put.
"The police have moved most of the protesters and they've told me that if I refuse to detach myself from the gate, that I'll be arrested, Dr Searle told AAP.
"I am refusing to leave."
The blockade follows a run of stop-work orders imposed on the Forestry Corporation over its alleged incompetent conduct in state forests that are home to endangered species, including Australia's largest gliding possum.
Dr Searle believes many Australians don't understand the destruction being done in native forests by a corporation owned by the NSW government.
"People are not aware of how absolutely disgraceful the behaviour of the Forestry Corporation is in these areas."
AAP has sought comment from the Forestry Corporation.
The Environment Protection Authority has stopped harvesting in two state forests in recent weeks, amid allegations of incompetent conduct.
The watchdog has raised serious concerns about pre-harvest surveys that are meant to identify and protect habitat trees for at-risk species in the Tallaganda and Flat Rock state forests.
In both cases, concerned citizens went into those forests and found hollow-bearing den trees that the Forestry Corporation did not.
The corporation has admitted it went looking for Southern Greater Glider den trees in Tallaganda during the day, when the nocturnal greater gliders would have been asleep inside their tree hollows.
The EPA is now revising protocols to ensure the forestry industry conducts searches in a competent way.
But the watchdog is refusing to release a photo its officers took of a dead, endangered Southern Greater Glider that was found about 50 metres from where trees were being felled in the Tallaganda State Forest.
Under rules designed to protect important habitat, the Forestry Corporation must identify den trees and enforce a 50-metre exclusion zone to safeguard the bush that surrounds them.
NSW Greens MP Sue Higginson says members of the public need to see the photo and the results of an autopsy.
"It's a public interest matter," she said.
"As upsetting as these images may be to some people, they are fundamental in the public being informed about what's happening to nature on the public forest estate."
Late last week, the EPA said: "We do not intend to release the post-mortem or images of the dead glider to avoid the risk of prejudicing any legal investigation processes."
At budget estimates hearings earlier this month, EPA chief executive Tony Chappel said investigations had identified a number of potentially very serious breaches by the Forestry Corporation in Tallaganda.
Mr Chappel said he had preliminary advice about the glider's cause of death, but wouldn't release it to avoid prejudicing any legal process that may eventuate.
Asked if that information would eventually be shared with the public, he said: "At the relevant point in time, absolutely."
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