A Muslim cleric charged over sending offensive letters to the families of dead Australian soldiers has lost his bid to have the charges quashed.

Man Monis, who is also known as Sheikh Haron, is charged with 12 counts of using a postal service in an offensive way.

The charges relate to letters and CDs he allegedly sent to the wives and parents of Australian soldiers who have been killed in Afghanistan.

Monis was seeking to have the indictments quashed in the New South Wales Court of Criminal Appeal on the grounds they were unconstitutional.

Lawyers representing Monis had argued the charges impinged on their client's implied freedom of political communication.

They said the letters concerned political affairs including Australia's foreign policy.

But a three-judge panel today rejected that argument.

The judges said that while the letters were critical of Australian military involvement in Afghanistan, they also referred to the dead soldiers in a denigrating and derogatory fashion.

The matter is now due to return to the NSW District Court.