At the start of World War I, "Pioneer" sailed from Victoria to Western Australia, where she served as a patrol vessel. On 16 August, she captured the German merchant ship "Neumunster", which was taken by the Australian government as a prize of war and renamed "Cooee". Ten days later, the cruiser captured a second German ship, the Norddeutscher Lloyd liner "Thuringen", which was presented to the government of India for use as a troop transport. On 1 November, "Pioneer" joined the escort of the convoy transporting the Australian and New Zealand Army Corps to Egypt as a replacement for the Japanese cruiser "Nisshin", and with orders to check on the Cocos Islands during the voyage. However, as the Australian warship took up position, she suffered a major engine malfunction, and joined the convoy instead.