[2] Nevertheless, the battalion's first major action did not come until mid-1917 when the British shifted their focus towards the Ypres sector in Belgium and the 3rd Division was committed to the Battle of Messines as part of II Anzac Corps on 7 June.

[7] Beginning their approach march late on 6 June, they were caught in a German gas bombardment that inflicted heavy casualties; nevertheless, they arrived at the line of departure on time and, after a number of mines were exploded in front of their positions, the assault began.

By 5 am on 7 June, the division had gained the crest of the Messines Ridge and dug-in to defend against a possible counter-attack,[7] with the 34th occupying a position around Grey Farm.

[11] Striking significant blows against the British southern flank, the Germans pushed the Allies back towards the vital Amiens railhead.

[12] The 34th Battalion subsequently took part in heavy fighting when the Australians counter-attacked at Hangard Wood on 30 March, before finally turning back the German attack on Villers-Bretonneux on 4 April.

[2] The involvement of the Australian Corps in the earlier actions in 1918 had significantly depleted its battalions which had suffered heavy casualties that they had been unable to replace as the number of volunteers arriving from Australia had fallen.

As a result, upon Australian Prime Minister Billy Hughes' request, it was subsequently withdrawn from the line for reorganisation and rest in October.

[20] In 1927, when territorial designations were introduced, the battalion adopted the title of the "Illawarra Regiment" and the motto Malo Mori Quam Foedari.

[18] Initially, a mixture of voluntary and compulsory service was used to maintain the strength of the Citizen Forces units; however, in 1930, the newly elected Scullin Labor government abolished the Universal Training Scheme, replacing it with a volunteer-only Militia.

[21] During World War II, due to the provisions of the Defence Act, which precluded Militia units from being sent outside Australian territory to fight,[22] the battalion remained in Australia where it undertook defensive duties variously assigned to the 14th and 28th Brigades.