The trees are proposed to be removed as part of highway widening works between St Helens and Basin Creek Road, but the development application was yet to be considered by Break O'Day Council.

The department claimed that covering over the hollows in advance of the works had been undertaken "based on specialist advice from ecological consultants" to reduce the risk of breeding disturbances later during the project.

But BirdLife Tasmania convenor Eric Woehler said the department was making an assumption that its plans would be passed by council in the current form.

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"They're making the assumption that there won't be any appeals, but clearly from the response from the community to what's happening, there will be some form of appeal against the development application," he said.

"If they move the road a handful of metres to one side, they wouldn't need to cut down the eucalpyts that have the nesting hollows.

"An overtaking lane a few kilometres south of St Helens is hardly necessary for destroying habitat for a critically endangered species."

While BirdLife had not seen the covered hollows firsthand, it had been provided information that plywood had been nailed across the hollows. The government confirmed that "initial tree treatment works" had been undertaken.

Swift parrots nest in different parts of the East Coast each year, relying on hollow-bearing trees near appropriate trees with sufficient flower. Because they take a different route each year, they rely upon tree hollows throughout Tasmania's east.

A government spokesperson said it was a "proactive" way of preventing potential harm to swift parrots.

"Covering tree hollows is a common pre-construction practice that presents considerable conservation benefits, as it reduces the risk of breeding disturbance during later stages of a project," the spokesperson said.

"No fauna was present in any of the hollows, and many of the hollows were considered potentially unsuitable for nesting based on size."