Tonelli took up swimming due to his asthma, and quickly came to prominence. Selected to represent Australia at the 1973 World Championships, he came sixth in the 200 m backstroke at the age of 16. He won his first Australian titles in 1974 in the 100 m backstroke and 200 m butterfly and went on to the 1974 Commonwealth Games in Christchurch, where he won his first major international race, the 100 m backstroke, and took silver in the 200 m backstroke. In 1975, Tonelli won his only individual medal at global level, a silver in the 200 m backstroke at the World Championships in Cali, Colombia. In 1975, Tonelli enrolled at the University of Alabama in the United States, studying and competing in the collegiate sport system. He was selected in both backstroke events for the 1976 Summer Olympics in Montreal, but struggled and missed the medals in both events. During his stay in America, Tonelli set times that would have placed him among the world's leading swimmers, but he was expelled from the 1978 Commonwealth Games team for breaches of discipline.