The advice claims environmental impacts must be assessed first.

Greens leader Bob Brown presented the legal opinion on the proposed mill construction, which was provided pro bono for the party, yesterday.

He said that, contrary to Federal Government assurances, environmental lawyer Vanessa Bleyer advised that construction of the Tamar Valley pulp mill could not begin before all issues affecting threatened and migratory wildlife species had been assessed.

''All issues associated with protection of the marine environment must be also addressed before the mill is commissioned,'' he said.

Environment Minister Peter Garrett approved construction of the mill on Monday, but said it could not operate until after the impact of effluent on the marine environment was tested. Gunns has two years to do the testing.

A spokesman for Mr Garrett rejected the Greens' assertion yesterday. He said Mr Garrett had departmental and legal advice that Gunns could start work on all aspects of the project which the Federal Government had approved, including the construction. AAP