Australia wanted the United Nations to play a bigger role in the reconstruction of Iraq after the 2003 invasion but was rebuffed by the Bush administration, a declassified report reveals.

The 572-page report written by a senior scholar at the Directorate of Army Research and Analysis and obtained under freedom of information laws says the then Howard government even discussed limiting Australia's military contribution to the post-invasion phase to positions under the UN.

The revelation is significant because the United States' poor post-invasion planning is widely blamed for the downward spiral Iraq plunged into which is still being felt today in the country's sectarian fracturing and the success of the Islamic State terror group.

"Australia also had a preference for the United Nations to have a large role in the rehabilitation of post-war Iraq," the report by Dr Albert Palazzo states.