Leech's health improved, but she remained slight and frail, with a double curvature of the back. In an effort to help her gain muscle control and confidence, her parents sent her to ballet, before trying swimming, a sport her mother had competed in. Aged six, she was taken to swimming classes while the family holidayed in the Mornington Peninsula. Leech was coached by Gus Froelich, a former European swimming champion and coach of Australian Olympic medallist Judy-Joy Davies. After a difficult start, Leech improved in her second year. At the Victorian Championships, she showed her potential by setting a state record of 17.4 seconds (s) for the 25 yd freestyle in the under-8 division. The following year, she covered 25 yd in 15.7 s, three seconds faster than Davies had done at the same age. She progressed steadily, sweeping the state age titles from nine to 13, setting records that were often faster than those by boys of the same age.
Living in Bendigo and studying at Camp Hill Primary School, Leech could only travel three times a year to train with Froelich, so she relied heavily on dry land simulations, such as a pulley attached to the kitchen door. When she was 12, she covered 110 yd in 1 minute (min) 7.1 s, setting an unofficial world record for her age group. This achievement prompted her parents to rent an apartment in Melbourne, so she could train with Froelich on a regular basis. Leech moved there with her mother, while her father stayed to look after the family's jewellery business. From 1954, Leech attended St. Michael's Girls' Grammar School and trained with Froelich on a daily basis at the City Baths. The regular sessions paid off at the 1955 Victorian Championships; Leech won the open 110 yd and 220 yd freestyle in times of 1 min 7.2 s and 2 min 39.3 s respectively, setting state records in both events. She proceeded to the Australian Championships, winning the 110 yd freestyle in 1 min 7.6 s to become the youngest ever winner of an open title at the age of 13. Leech also won the national junior 110 yd and 220 yd events.
Leech's regimen differed from most swimmers because Froelich was not an advocate of distance training, and designed shorter workouts for his