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.

At the age of 12, Hayley entered a professional recording studio to record Walking in the Air, a demo album as a memento for friends and family—only 1000 copies were pressed. With her album finished, Hayley went with her sister Sophie, also a singer, to busk in the Christchurch streets. The pair drew a large crowd, and one woman asked the girls if they had recorded anything. This woman turned out to be a journalist with CTV, and asked Hayley to appear on air. A concert promotion company watched the show, and signed her to the Universal New Zealand Label. On this label, Hayley released a self-titled album of show tunes and light classical songs, and My Gift to You, a CD of Christmas music.

Although her albums were quite successful in New Zealand, Hayley was virtually unknown worldwide until she landed a contract with the Decca Music Group, and recorded Pure, a CD of classical, light pop, and traditional Maori songs.

Pure, her first album on the international market, enjoyed record-topping success. It became the fastest selling debut album in the history of the UK classical charts, beating the previous record of child soprano prodigy Charlotte Church. Pure sold 19,068 copies in its first week alone, flew to Number 1 on the UK classical charts, and entered the UK Pop Chart at #8 (UK Chart data courtesy of The BBC). To date, over two million copies of Pure have been sold.

Recently, Hayley has done a special concert for Prime Minister Tony Blair, President George W. Bush and Her Majesty The Queen. Hayley is also the youngest UNICEF Ambassador to date, and visited Ghana to distribute bicycles to girls so that they could go to school. Some time after releasing Pure, Hayley and her family moved to London, England.

Hayley just recently finished recording the end-title song for Disney's movie Mulan 2. She took her first acting role on Episode 46 (Charade) as a Singer in the Lair on the TV show American Dreams (NBC) where she played the guitar and sang "Who Painted the Moon Black?". She also had a special air on PBS' Great Performances. This special was entitled, "Hayley Westenra: Live from New Zealand."

Westenra is the 2004 Vodafone New Zealand Music Awards winner of "Highest Selling New Zealand Album" and "International Achievement Award" She's won two Japanese Grammies for her work. Also in 2004 she began her world tour of New Zealand, Australia, Japan and the UK. While she was preforming at the Sydney Opera House she received a rare standing ovation.

Recently, Hayley released a new album called Odyssey, titled for her journey to musical success. A mix of classical, pop, Celtic, and new age, Hayley has co-written and arranged some of the tracks on the album, which was released in the US on October 18.