This website (http://en.allexperts.com/e/a/al/allies_of_world_war_i.htm) says the following:
"Although the Dominions and Crown Colonies of the British Empire made significant contributions to the Allied war effort, they did not have independent foreign or defence policies during World War I. The Dominion governments controlled recruiting in their countries, and while they did not have operational control of their personnel, they did remove personnel from front line duties as they saw fit. Overall control was in the hands of the five-member British War Cabinet (BWC). From early 1917 the BWC was superseded by the Imperial War Cabinet, which had Dominion representation. Late in the war, the First Australian Imperial Force and Canadian Expeditionary Force were grouped in their own separate army corps, under the command of Australian and Canadian generals, who reported in turn to British generals."