The decision reverses the step towards transparency that the agency took three years ago, when it became the first of many federal departments to release its ''red book'', the introductory brief for an incoming Labor minister.

Late on Friday, the Treasury said it would reject requests to access the Coalition brief, or ''blue book'', which it gave to Mr Hockey last month. It said making the secret brief public might damage its relationship with the new Treasurer.

The document is a comprehensive presentation of the main economic and policy challenges facing Australia and the federal budget.

The Treasury never released the full version of its red book three years ago, deciding instead to publish a censored version in response to freedom of information requests.

However, it now says the briefings contained within the blue book ''are amongst the most important and highly confidential documents produced by the department. If that confidentiality cannot be guaranteed, then future incoming government briefs are unlikely to be as comprehensive and, therefore, less useful to a new government in the initial stages of its administration.''

The Treasury also argued it must protect its relationship with the Treasurer. ''The need to develop a trusting relationship is particularly important in the early days of a new government, to set the tone for the future working relationship of the whole department,'' it said.

Australian Information Commissioner John McMillan predicted in 2011 that all red and blue books would be released in future after the Treasury had taken the groundbreaking step to do it. ''Once you've got to a level of transparency, you can't undo it,'' he said at the time.

The Coalition, when in opposition, regularly criticised Treasury secretary Martin Parkinson, who is leaving the department next year.

Prime Minister Tony Abbott announced Dr Parkinson's pending departure last month.