 Trethowan entered parliament at the 1980 state election, replacing Sir Des O'Neil (a former deputy premier).He was elected unopposed, as the Labor candidate failed to submit his nomination on time.At the 1983 state election, Trethowan won his seat with 53.6 percent of the first-preference vote, defeating candidates from the Labor Party (Carmen Lawrence, a future premier) and the Australian Democrats (Shirley de la Hunty, a former Olympic athlete).