Westenra is the child of Jill and Gerald Westenra; and has two siblings, Sophie and Isaac. Hayley comes from a musically-inclined family; her grandmother was a singer and her grandfather was a pianist and played the piano-accordion. Hayley is of Welsh and Irish ancestry.

Her musical career began at the age of six when she was cast the role of "the littlest star" in her school's Christmas play, with a singing part. After the show, a teacher who had seen Hayley sing approached her parents and informed them that the girl had perfect pitch. (In fact, she is sometimes referred to as a "female Josh Groban".) She encouraged Hayley to pick up a musical instrument to improve her abilities, and Hayley began to take violin lessons. Along with violin, she took up the piano and recorder, and learned to read music by the age of 7. She began voice lessons and discovered a passion for musical theatre. By age 11, Hayley had performed more than 40 times on stage.