Following her departure on Australian Idol, Paulini signed a recording contract with Sony Music Australia, which later merged with the show's sponsor BMG Records. Her debut studio album One Determined Heart was released on 23 July 2004; it debuted at number one on the ARIA Albums Chart, where it remained for two consecutive weeks, and was certified platinum by the Australian Recording Industry Association (ARIA) for shipments of 70,000 units. The album was produced by Audius Mtawarira, and was mainly a collection of covers, featuring only two original tracks. A cover of The Jeff Healey Band's 1989 hit "Angel Eyes" was released as the first single from One Determined Heart. It peaked at number one on the ARIA Singles Chart, where it remained for three consecutive weeks, and was certified platinum. "Angel Eyes" remained in the top-ten for two months, and became the fourth highest-selling Australian single of 2004. Paulini won two ARIA No. 1 Chart Awards for both the album and single. "Angel Eyes" was also nominated for Highest Selling Single at the 2004 ARIA Music Awards. Original track "We Can Try" was released as the album's second single, and peaked at number 30. The song had better success on the ARIA Urban Singles Chart, where it reached number nine.
In July and August 2004, Paulini was the supporting act for Human Nature's seven-date tour on the East Coast. Paulini's first extended play, Amazing Grace: Songs for Christmas, was released on 26 November 2004, featuring covers of popular Christmas tunes. The EP peaked at number 70 on the ARIA Albums Chart and number seven on the ARIA Urban Albums Chart. That same month, Paulini served as a supporting act for Tina Arena's Greatest Hits Tour. She was also the supporting act for Guy Sebastian's Beautiful Life Tour from March to June 2005.
On 26 November 2004, Paulini made a guest appearance in the Australian soap opera Home and Away as herself. In the episode, Paulini performs her single "We Can Try" at the opening of Summer Bay's new bar Noah's, and later performs "Silent Night" at the christening for Sally Fletcher's (Kate Ritchie) baby. Paulini appeared as a contestant in the first season of the Australian lifestyle reality series Celebrity Overhaul in February 2005, alongside six other celebrities. On the show, celebrities went through a six-week course of fitness and lifestyle retraining. In May 2006, Paulini joined the first season of the celebrity singing show It Takes Two, where 10 non-singing celebrities are paired up with 10 professional singers to compete in a duet-singing showdown in order to impress a panel of judges and the viewing public to survive potential elimination. Paulini was paired up with television presenter Simon Reeve, and they placed sixth in the competition. For the second season of It Takes Two in May 2007, she was paired up with television presenter Lochie Daddo and they were the first couple to be eliminated from the competition. Paulini made her acting debut in the television film Sisters of War, which premiered on ABC1 on 14 November 2010. The film is based on the true story of a group of Australian nuns and nurses held by the Japanese as prisoners of war at the Catholic mission station Vunapope, on the Papua New Guinea island of New Britain in 1942. Paulini played the supporting role of islander nun Sister Marie, who she described as caring and strong. In preparing for the role, Paulini spoke with nuns who had been through World War II experiences and read books on one of the film's other characters, Sister Berenice Twohill. During one scene in the film, Paulini's character sings a Latin requiem in Papua New Guinean dialect while standing beside a huge cross, after death has come to the mission during an aircraft attack. In 2019, Paulini appeared as the Spider contestant in first season of The Masked Singer Australia, being eliminated in finals week.