In 1975, Tonelli won the 100 m backstroke, 200 m backstroke and 200 m butterfly events at the Australian Championships. Although he won more individual titles than in the previous year, his times were slower; he completed the distances in 59.70 s, 2 m 10.50 s and 2 m 10.00 s respectively. Tonelli was also part of the Queensland team that successfully defended their medley relay title. Tonelli was thus selected for the 1975 World Championships in Cali, Colombia. He qualified fastest for the 200 m backstroke final and had planned to pursue an aggressive strategy to attack from the start. However, he was advised to swim conservatively in the first half of the race by Australian coach Terry Gathercole so that his main rivals Matthes and John Naber would not be able to draft behind him in the early stages. The tactic backfired and Tonelli ended with the silver medal. He had swum faster in the second half of the race—something exceedingly rare in top-level swimming and an indication of strategic error—and felt that he had too much unused energy left at the end of the race. Tonelli vowed that from then on, he would always back his judgment and race strategy.