P02474.001 Newcastle, NSW. September 1916. Group portrait of buglers of the 15th and 16th Infantry (Newcastle Battalion) Broadmeadow Camp. 

The best analogy to explain the structure of the Army is a collection of building blocks called sub-units that are combined in different ways to form larger groups that are known as units. Different combinations of these units can in turn be put together to form larger groups, known as formations. The basic structure of the Army is outlined below:

Basic Army structure
Army sub-units and units by corps
Infantry Division: 1916
Infantry Division: 1941

Basic Army structure

Formations Organisation Strength Made up of Commanded by
Army   Two or more corps General
Corps 30,000 or more Two or more divisions Lieutenant General
Division 10,000 - 20,000 3 brigades Major General
Brigade 2,500 - 5,000 WWI WWII Brigadier (General)
4 battalions 3 battalions
(sub-) units Battalion 550 - 1000 4 companies 4 companies Lieutenant Colonel
Company 100 - 225 4 platoons 3 platoons Captain or Major
Platoon 30 - 60 4 (later 3) sections 3 sections Lieutenant
Section 9 - 16     Corporal/Sergeant

Formations consist of a variety of different corps units in addition to the infantry. The different structures of these other units are shown below. The sub-units are listed in ascending order.

Army sub-units and units by corps

 
Corps
Sub-Unit & commander
Unit & commander
Army Service Corps
Engineers
Infantry
Intelligence
Military Police
Ordnance
Section (Sect) Corporal Battalion (Bn) Lieutenant Colonel
Platoon (Pl) Lieutenant
Company (Coy) Captain or Major
Aviation
Cavalry/Armour
Engineers (post-1945)
Signals
Transport
Section (Sect) Corporal Regiment (Regt) Lieutenant Colonel
Troop (Tp) Lieutenant
Squadron (Sqn) Captain or Major
Artillery (pre-1939) Detachment (Det) Sergeant Brigade (Bde) Lieutenant Colonel
Section (Sect) Lieutenant
Battery (Bty) Major
Artillery (post-1939) Detachment (Det) Sergeant Regiment (Regt) Lieutenant Colonel
Section (Sect) Lieutenant
Troop (Tp) Lieutenant/Captain
Battery (Bty) Major

Australia has only fought with forces larger than a brigade in the two world wars. The diagrams below roughly illustrate the organisation of an Australian infantry division in 1916 and in 1941.

Infantry Division: 1916

Infantry Division: 1941