An environment group fears the state-owned logger is leaving giant trees nearly six meters wide off its protection register.
Forestry Watch says it's identified five huge Mountain Ashes in the Huon Valley which are in a contingency area that could be logged anytime.
Dr Jennifer Sanger from Forestry Watch says there's no mention of the trees in Sustainable Timbers Tasmania's Forest Practices Plan for the area.
"These five trees are actually in one of the logging coupes selected by Sustainable Timbers Tasmania," she said.
"What we're hoping for is that STT will look at these trees and make a reserve to protect them.
"It is really concerning that these trees have not been mentioned in their Forest Practices Plan, it is pretty difficult to miss five giant trees and it makes me wonder if these preliminary surveys are being done thoroughly."
STT has been contacted for comment.
Top: A 5.74-metre wide giant in the contentious area (supplied)
Bottom: a 4.5-metre wide Eucalypt logged by STT (Southern Forest Alliance).