On 28 January 1915, Leak enlisted in Rockhampton as a private in the Australian Imperial Force (AIF) and was assigned to the fifth reinforcement draft for the 9th Battalion, 3rd Brigade, 1st Division. On 16 April, he embarked from Brisbane, Queensland, on board the transport "Kyarra". He joined his battalion on 22 June while it was involved in trench warfare defending the Anzac Cove beachhead on the Gallipoli Peninsula in the Ottoman Empire. This fighting was part of the Gallipoli Campaign. Six days after Leak's arrival, the 9th Battalion was involved in a two-pronged attack against the Ottoman positions on what were known as "Sniper's Ridge" and "Knife Edge", close to Lone Pine. The attack was abortive, and resulted in the battalion suffering casualties of 37 killed and 62 wounded. Leak was with his battalion during the fruitless August Offensive, which was intended as a breakout from the beachhead. On 31 August, Leak reported sick and was evacuated with gastroenteritis, first via hospital ship to Malta, where he was hospitalised and diagnosed with dysentery. In late September he was evacuated to the United Kingdom where he was again hospitalised. He did not return to his unit until 23 February 1916, by which time it, along with the rest of the AIF, had been withdrawn back to Egypt for re-organisation.