Born in 1920 in suburban Melbourne, she had hoped as a girl to become a ballerina but switched to singing when her teenage body developed too heartily for dance. After vocal studies in Melbourne, she traveled to London to work with the tenor Joseph Hislop, who also coached Jussi Bj‹rling and Birgit Nilsson.
Her roles as a rising young soprano in Australia ranged from Mozart's Cherubino, Fiordiligi and Donna Anna to Puccini's Butterfly and Tosca to Wagner's Senta and Elsa. While touring the country in Tosca, she fell in love with her Scarpia, Robert Simmons, and decided to marry him while they were in Perth — though both were already married at the time, quite the scandal in 1950s Australia.
They went to Great Britain together in 1954, and Fretwell eventually became a key member of the Sadler's Wells company, where she received critical praise for her vocal and dramatic skills in a number of Verdi and Puccini leading roles and as Strauss's Ariadne. She sang at the Royal Opera House only a few times in 1965, as Aida and Giorgetta (Il tabarro). She returned to Australia permanently in 1970, sang in the opening season of the Sydney Opera House in 1973 and remained a member of the Australian Opera (now Opera Australia) until she retired.