Andrew volunteered for the New Zealand Expeditionary Force (NZEF) in October 1915, when he was 18. As only men between the ages of 20 to 40 could volunteer for service with the NZEF, he falsified his age to ensure that he would be eligible for duty overseas. A member of the 12th Reinforcements, he embarked for the Western Front via Egypt on 1 May 1916. In France, he was posted to B Company, 2nd Battalion, Wellington Infantry Regiment, with the rank of private.