The Council of Trade Unions is considering going to court to get a copy of the Crown Law opinion on last year's industrial dispute over the filming of The Hobbit.
In a final decision, Ombudsman David McGee has rejected appeals from the CTU and Radio New Zealand and says the legal opinion should remain secret.
Last year, the Government said the Crown Law opinion confirmed that by law, film producers could not enter into union-negotiated agreements with actors who were independent contractors.
Mr McGee says the opinion was given to US film studio Warner Brothers, but that was done under the understanding the company would respect its confidentiality.
The Council of Trade Unions says it will seek legal advice about challenging Mr McGee's decision.