Sister Mary Leo was New Zealand’s best-known singing teacher in her time. She was born Kathleen Agnes Niccol at Devonport, Auckland, on 3 April 1895, the eldest child of Henry Malcolm Niccol, a shipping clerk, and his wife, Agnes Theresa Cannell. Her father was from a leading Devonport family, and from her mother's side she inherited her musical talent and acquired a deep devotion to the Catholic faith.
Kathleen's musical abilities were nurtured by her family and piano lessons began when she was about eight. She first attended St Leo's Convent, Devonport, a Sisters of Mercy school close to the Niccol home. In 1906 the family moved to the city side of the harbour and Kathleen completed her schooling at St Joseph's Convent School, Grey Lynn. Here she had music lessons from Sister Cecilia Cavalier, of the Sisters of St Joseph of the Sacred Heart.
In 1912 Kathleen Niccol was a first-year pupil-teacher at Richmond Road School in Grey Lynn. Later, she taught for four years at Onerahi School near Whangarei. When she left in 1919 the headmaster described her as a 'remarkably gifted' teacher. Her next teaching post was as governess to Leslie and Eva Wilson’s family on their farm near Waipukurau in Hawke's Bay. Kathleen then lived with Leslie and Eileen Harker and their family in Waipukurau for about two years while running a small private school in a nearby home. She also took dancing classes and organised recitals.
Kathleen Niccol had decided as a teenager to be a nun, but did not join the Sisters of Mercy until she was 28. She entered St Mary's Convent, Ponsonby, Auckland, in May 1923 and took her final profession of vows in 1929. She also received further musical training, passing her LTCL in piano in 1925 and teacher's diploma in singing in 1927. Now known as Sister Mary Leo, from 1930 she was in charge of music at St Mary's College.
St Mary's already had a strong reputation for success in music exams. Under Sister Mary Leo's leadership, and with the help of colleagues like Sister Mary Xaveria (later Sister Mary Francis Xavier), the music department grew and developed. By 1949 St Mary's College School of Music, as it was now called, occupied a two-storeyed wooden house, called Stella Maris, with several assistant teachers and 17 teaching or practice rooms. Piano, violin, cello and theory were taught, but Mary Leo also increasingly took private singing pupils, in classes and individually. Her St Mary's choir became well known for its beautiful sound and her singing pupils won prizes at local competitions.
In 1950 a pupil, Mina Foley, came second in one of Australia’s most prestigious competitions, the Melbourne Sun Aria, shortly after she had won the John Court Aria in the Auckland competitions. Mina was Mary Leo's first star pupil and, more than any other, established her reputation as an outstanding teacher. During the 1950s talented pupils, including Heather Begg, Kiri Te Kanawa and Malvina Major, sought out Sister Mary Leo as a teacher. At the same time pupils like Elisabeth Hellawell, Mary O'Brien, Angela Shaw and Hannah Tatana were becoming well known through operatic performances within New Zealand. Mary Leo's pupils dominated the John Court Aria competition throughout the 1950s and 1960s, and won every Mobil Song Quest from 1959 until 1970. Many went on to win prizes in Australian competitions and contracts with opera houses overseas. The Melbourne Sun Aria was won by Malvina Major in 1964, Kiri Te Kanawa in 1965 and Lynne Cantlon in 1966.
In 1960 Mary Leo was included in a radio programme, 'This is New Zealand', featuring 13 prominent New Zealanders. In 1963 she was made an MBE. By now well known throughout the country, she also enjoyed a new personal freedom following the relaxation of some of the restrictions on the activities of religious. She liked attending concerts and supported her pupils at competitions. In 1967 she conducted her choir in public for the first time, and by the following year had adopted the calf-length habit and lighter veil which meant that some of her hair could be seen.
Full of vital and creative energy, Sister Mary Leo spoke and moved rapidly and had a quick, receptive mind. While she claimed to follow no particular method, but to use her own psychological approach which was individual to each pupil, it is possible to trace the influences on her teaching back to the Italianate school of García and Marchesi. The pure, floating quality of sound, commonly known as bel canto, typical of this school was a hallmark of her pupils' singing.
Sister Mary Leo worked long hours, often under pressure, when staging major concerts in the Auckland Town Hall or preparing pupils for competitions and examinations. A perfectionist, she was demanding and rarely gave praise, always expecting ‘nothing less than one’s best’. Some found her over-exacting and unsympathetic but most, especially those who came as mature students, found her warm and affectionate. She was sustained throughout her career by her deep religious convictions, an active prayer life and the support of her religious community. She saw herself as having been given 'a gift for creative training of voices' and also saw her pupils' voices as gifts from God.
In 1972 Sister Mary Leo made a world tour, visiting her former pupils and hearing them sing in leading operatic roles. In 1973 she was appointed a DBE. Talented pupils continued to seek her out, though by the late 1970s her teaching had lost its earlier vigour. She continued teaching until a fall resulted in a broken pelvis in 1985. From this time on she lived in the Mater Misericordiae (later Mercy) Hospital until her death there on 5 May 1989.