In 1973, at the age of 15, Tonelli competed in his first Australian Age Championships in Hobart, winning the 100 m and 200 m freestyle, and the 200 m backstroke. These results allowed him to swim at the preliminary qualifying trials for the 1973 World Aquatics Championships, where he managed four fourth placings. However, the selectors held another set of trials just before the World Championships, which offered swimmers a final chance to gain selection. Tonelli said "Everyone, except me, knew it was a ploy simply to keep the team on its toes".