It seems that everybody is allowed to write a book about themselves, unless our fine government dislikes them or their message. To quote from the Overland Blog, "On 20 July 2011, the Australian government served David Hicks with a notice of their intent to restrain any funds obtained from the sale of his book, Guantanamo: My Journey, under the Commonwealth Proceeds of Crime Act."
The Overland team have written more about this and have begun a Petition in Support of David Hicks which I have signed. I urge other people who believe in freedom of speech to sign it too—regardless of your feelings about David Hicks the man.