 When Britain declared war on Germany at the start of the First World War , the Australian government rapidly followed suit , with Prime Minister Joseph Cook declaring on 5 August 1914 that " ... when the Empire is at war , so also is Australia " and reflecting the sentiment of many Australians that any declaration of war by Britain automatically included Australia . This was itself in part due to the large number of British @-@ born citizens and first generation Anglo @-@ Australians that made up the Australian population at the time . Indeed , by the end of the war almost 20 % of those who served in the Australian forces had been born in Britain . 
