[3] Later, in July and August the 20th took part in the Battle of Pozières, during which time it served in the line twice before moving to a quieter sector in Belgium for a period of rest.

In October, however, the entire 2nd Division was moved to the south again and put back into the line in France once again and in November they launched an attack at Flers, in conditions that were so muddy that they were described by the official historian, Charles Bean, as "the worst ever encountered by the AIF".

[3] Throughout 1917 the 20th Battalion was involved in three major battles as the German Army was forced back towards the Hindenburg Line, seeing action at Bullecourt in May, Menin Road in September and Poelcappelle in October.

[3] At Lagnicourt, on 15 April, along with other units from the 5th Brigade, they took part in a defensive action along with four other battalions, where they managed to defeat a counter-attack by a German force almost five times its size.

[3] It was disbanded on 20 April 1919 while at Montigny-le-Tilluel, Belgium, when most of its personnel, including the commanding officer, Lieutenant Colonel Frederick Forbes, were marched out for repatriation to Australia as part of the demobilisation process.

[7] Territorial designations were introduced in 1927, at which point the battalion adopted the title of the "Paramatta and Blue Mountains Regiment"; at the same time the motto of Pro Patria was approved.

[10] In 1929, following the suspension of the compulsory training scheme by the newly elected Scullin Labor government,[11] financial pressures and manpower shortages resulted in a number of battalions being amalgamated.

[15] Following the outbreak of World War II, the 20th/19th Battalion was transferred from Sydney to Darwin in the Northern Territory, where they were to form part of the town's garrison.

[20] Nevertheless, as a result of a manpower shortage in the Australian economy, the government decided to disband or amalgamate a number of Militia units throughout 1942–43,[21] and on 13 December 1943, the 20th merged with the 34th to form the 20th/34th Battalion.