Rennie served with Jayforce, New Zealand's contribution to the occupation of Japan, between March and August 1946 before returning to instructional positions including a posting to the British Army School of Infantry and two and a half years as Chief Instructor of the New Zealand School of Infantry.
In 1955, Rennie was selected to form, train and command the New Zealand Special Air Service Squadron (NZSAS) for service in the Malayan Emergency.