 He worked as a draftsman when he entered Queensland PWD.He decided to leave in 1920 and started his own architecture design in Brisbane where he practiced for his entire life.Working during the interwar period, Trewern was involved in designing a variety of projects such as the Country Press building, Elizabeth and Edward Streets, Brisbane (1924-5), Heindorff House, Queen Street, Brisbane (1926) and Inchcolm, Wickham Terrace, Brisbane (1929).