Turnbull was an active campaigner in the unsuccessful 1999 referendum to establish an Australian republic, serving as Chair of the Yes Committee. He published a book on the campaign, titled "Fighting for the Republic". When the referendum failed, he accused incumbent Prime Minister and monarchist John Howard of "breaking the nation's heart". Turnbull retired from the Australian Republican Movement in 2000, having already left the board of Ausflag in 1994; he joined the Australian National Flag Association in 2004.