By the age of 10, Theile was enjoying success at local swimming carnivals, so his father offered to arrange for him to be coached, but left Theile with the choice of coaches. Of the two coaches in the town, Theile chose Arthur Cusack, who had a reputation as a physical training instructor of whom Theile recounted as being "tough and enthusiastic. All my friends avoided him...somehow I sensed that it was an important decision." Cusack turned out to be his lifelong coach. Under the guidance of Cusack, Maryborough Swimming Club challenged the metropolitan clubs in Brisbane for statewide supremacy. Theile won Queensland junior and open titles in all four strokes. In 1954, at the age of 16, he came second in the open backstroke, and after becoming the national junior champion that year, he decided to concentrate on the given stroke. In 1955, he captured the 110yd backstroke title at the Australian Championships in Adelaide. At the same time, Theile completed his secondary education and was awarded an academic scholarship to study medicine at the University of Queensland.