The Chief Ombudsman is backing Government secrecy over concessions it made seven months ago in a deal with Warner Brothers over The Hobbit movies.
Mr Key led negotiations with film studio executives last October after a dispute between actors and filmmaker Sir Peter Jackson.
The Prime Minister has refused to release advice he received, despite the deal costing taxpayers another $30 million and the Government pushing through urgency a law change to help settle the dispute.
Chief Ombudsman Beverley Wakeham says disclosing the information would promote greater accountability and thereby enhance respect for the law.
In her opinion, though, that doesn't override the public interest in protecting the Prime Minister's access to free, frank and confidential advice.