After completing a conversion course , Headlam was assigned to the Seaplane Squadron at Point Cook . No larger than a flight according to the official history of the pre @-@ war RAAF , Seaplane Squadron was part of No. 1 FTS and operated Supermarine Southampton flying boats and de Havilland Gipsy Moth floatplanes , among other types . During this posting Headlam was promoted to flying officer , on 1 July 1935 , and wrote a paper on national defence in which he suggested that with " strong air forces , naval forces ( including submarines ) , and fixed defences , Australia may be made practically invulnerable " . According to Air Force historian Alan Stephens , this paper " in effect , defined the ' anti @-@ lodgment ' concept which has been a persistent feature of RAAF strategic thinking " .