One of the administrative changes instituted by the Gorton government was to establish the Cabinet Office as a separate Department of State. This 'CO' (Cabinet Office) file series was created as a result and is the main series maintained for the administration of Cabinet Office business. The files have blue covers and bear a red security
classification and/or a yellow "Limited decal when appropriate. The contents are more extensive than those of the preceding "C" series (CRS A4940) as there are no supporting PM and C files, and consist of submissions, minutes of decisions, supporting papers, briefs, press statements, press cuttings and memoranda relating to matters considered by the Ministry, the Cabinet and Cabinet committees. The series was maintained after the re-amalgamation of the Cabinet Office with the Department of Prime Minister and Cabinet, until the conclusion of the McMahon Ministry.

 The register of these files, CRS A5885, cites only the 'CO' file numbers and title, making no reference to a submission or decision number, however the register of submissions, CRS A5870, contains this information.

All items in this series in the custody of Australian Archives as at January 1998 have been entered onto the item level database, ANGAM II.