War Diaries were placed in AWM4. A war diary is a day to day account of events at a headquarters, usually kept by some staff officer. He got to decide what was worthy of record. Some recorded what was served each day in the mess hut. The most interesting part for the historian is the appendices, which contain reports and other documents thought important enough to file.
From the files collected by the Military History Unit, Captain Bean and the folks at the War Memorial assembled two artificial series, known as AWM25 and AWM26. The first is filed by topic, the second chronologically. AWM26 covers only the Western Front; the records of the Gallipoli Campaign are filed in AWM25 under G for Gallipoli while those on Palestine are filed under L for Light Horse. The records relating to the Sinai campaign are with the Gallipoli records, an error uncorrected after 80 years. Bean wrote his history from AWM26.
The Second World War records, when they arrived in Canberra, were organised in much the same way, except that there is no equivalent of AWM26. The war diaries are in AWM52 and everything else in AWM54. Operational records are filed under O, with number blocks being allocated to different units.
At a later date, the individual files were entered into the computer system, ANGAM, later RecordSearch which replaced ANGAM in the 1990s. They can be searched on keywords but an exact match is required. In the early years of the 21st Century, a digitisation project developed, making some of the war diaries available in PDF (Adobe Acrobat) form online. Each page of war diary is one electronic document. I use a C# program to take the diaries and merge the pages back into diaries again. Then I burn them onto CD.
Occasionally, one will find something outside these series. But not often. AWM52 and AWM54 provide most of the thesis. I have made a little use of interviews and comments by the participants in the official historians' records.