The federal permit issued to collapsed timber company Gunns in 2011 specified the project use plantation timber only, according to the company's own undertaking.

But Senator Whish-Wilson said a recent report by former federal Labor minister Martin Ferguson about the production capacity of Tasmania's plantation forests estimated they would yield up to three million cubic metres per annum, meaning stocks would fall 12.5 per cent short of the 4.5 million tonnes Gunns estimated the mill would need each year.

"I am concerned that once a pulp mill is established, the owners will lobby to be allowed to source their feedstock from Tasmania's native forests," Senator Whish-Wilson said.

"The pulp mill simply won't be able to source enough local plantation feedstock to be viable."

Senator Whish-Wilson plans to move a motion in the Senate today calling on the government to maintain the plantation-only condition if the permit is bought by a new investor.

Environmental Defender's Office of Tasmania principal lawyer Jess Feehely said the permit could be varied at any time with the consent of Federal Environment Minister Greg Hunt.

Ms Feehely said Mr Hunt would only be able to approve a variation if it would result in an "equal or improved environmental" outcome, but said it was unclear what that could be taken to mean.

"That's a concern, that there's that possibility that providing the condition requiring the minister to demonstrate the test for variation had been met, it could be changed without a requirement for it to be advertised," she said.

A spokesman for Mr Hunt said the existing permit stands and it was not a matter under consideration.