While Australia was distant from the main theatres of World War I, small military forces were maintained to defend the country from attack throughout the war. German raiders were considered the main threat, though there was also concern about acts of sabotage.
Background
In the years before World War I, the Australian Government began the process of raising a large civilian militia to defend the country against a feared attack by Japan. This force was based on conscription, and was intended to be complete in 1920. The resources devoted to this plan greatly exceeded those allocated to preparations to raise an expeditionary force to serve outside Australia.
The Australian Government also expanded the pre-Federation network of coastal defences to provide protection against raids from Japanese or German warships prior to World War I. In 1912, these defences were manned by 14 companies of the Australian Garrison Artillery, each of which had a strength of over 100 men.
Garrison forces
Mobilisation during 1914
All coastal defence batteries were activated hours before the declaration of war against Germany (which took place at 9:00 am Melbourne time on 4 August). The next morning, a 6-inch gun from the examination battery at Fort Nepean, which was located at the opening of Port Phillip Bay, fired a shell over the bows of the German cargo ship SS Pfalz after its Australian pilot failed to notice a signal for the ship to stop. Pfalz subsequently steamed into nearby Portsea, and was secured by a naval boarding party. This was the only shot to be fired at an enemy vessel in Australian waters during the war, and may have been the first shot fired by the armies of the British Empire during the war.
The militia members of the garrison units received call-up notices on 6 August, and reported for duty at 9 am the next day. During the early days of August all coastal fortifications were fully manned by gunners, engineers and infantrymen. The gunners remained at their stations for the rest of the year, as it was feared that the German armoured cruisers Scharnhorst and Gneisenau could attack Australia. In addition to the coastal defences in Australia, a battery of four 4.7 inch guns manned by naval personnel was established to protect the town of Rabaul in New Britain in March 1915; this town had been captured from German forces in September 1914.
After the initial call-up of reservists to man the garrison artillery, the commanding officers of the other militia units were directed to begin preparations to raise an infantry division and a light horse division for home defence tasks. By the end of August 1914 the Government had judged that a force of this size was unnecessary given that Australia was distant from the war zones. Instead, it was decided to maintain only a small number of infantry battalions and light horse regiments on active duty at any one time. By the end of 1914 the home defence force comprised 100,000 reservists, of whom 56,000 were members of the Citizens Forces and 51,000 were rifle club volunteers. Despite fears of sabotage and uprisings by German-Australians, no domestic threat eventuated. However, on 1 January 1915 twenty members of the 82nd Infantry Battalion assisted the police to kill two men who fired on picnickers during the "Battle of Broken Hill".
Members of the coastal artillery units were not initially permitted to volunteer for overseas service in the Australian Imperial Force (AIF). However, the threat of naval attack from German raiders greatly decreased after the British victory in the Battle of the Falkland Islands on 8 December 1914, and in early 1915 the Australian Government decided to form a heavy artillery brigade (which was eventually designated the 36th (Australian) Heavy Artillery Group) for service in France from the permanent members of the garrison artillery units.