 Following Britain 's declaration of war on Germany on 4 August 1914 , Australia and the other members of the British Empire became automatically involved , with Prime Minister Joseph Cook stating on 5 August 1914 that " ... when the Empire is at war , so also is Australia . " Given the predominantly British heritage of most Australians at the time , there was considerable support from all corners of the country and large numbers of young Australian men reported to recruiting centres around the country to enlist in the following months . When Prime Minister Andrew Fisher 's Labor Party came to power in September 1914 , he reiterated Cook 's statement saying , " Should the worst happen ... " , Australia would " ... rally to the Mother Country ... to help and defend her to our last man and our last shilling . " 
