Trigg joined the Royal New Zealand Air Force (RNZAF) as a trainee pilot on 15 June 1941. On enlistment, his occupation was recorded as "machinery salesman" and he was married with two sons, having married Nola McGarvey in 1938. After completing basic training at the RNZAF base at Levin, Trigg attended pilot training school at No. 3 EFS in Canada. Noted for his hard work and willingness to learn, he was recommended for a commission. He obtained his pilot's wings on 16 January 1942, and was commissioned as a pilot officer. After converting onto the Lockheed Hudson and completing further training at a reconnaissance school, Trigg was promoted to flying officer and embarked for the UK in October 1942, to join Coastal Command.