This series covers correspondence relating to specifically war-time  Defence matters (other than matters relating to AIF for which there was a separate series: B539). Topics dealt with include expeditionary forces, relationships with foreign governments, national resources, prisoners of war, intelligence reports, home defence, censorship, protection of shipping, inventions, and treatment of aliens. Registered correspondence was given a Department of Defence stamp with a W prefix to the registration number. Files were apparently kept in grey Department of Defence jackets, but many papers without related correspondence, and some files, were transferred to Archives without jackets and have been placed in folders. This series contains unclassified, confidential, and secret correspondence. The numbering system was based on that used for the 1913-1917 correspondence series (A2023) but as this was a more specialized correspondence series some of the primary subject divisions allowed for in that system did not have to be used. It was a three number system with a primary number allocated to major subjects, a secondary number allocated to divisions within the subject, and a third number allocated to papers
consecutively upon receipt. Each paper received was registered. If it related to previous papers it would be placed with them to make a file and was put away according to the latest number. When looking for an individual paper it is usually essential to refer to the
registrationbooklets (B544) which records these attachments.
This series was subject to heavy culling by the department in the late 1920's and again in the early 1950's. The registration booklets (B544) give some guide to these destructions, stamps having been placed on some entries to the effect that the papers had been destroyed or transferred to Archives. No files, other than those held by the Victorian Branch, are known to be extant.