2003–2007: Just As I Am, Beautiful Life and Closer To The Sun
In May 2003 Sebastian successfully auditioned for Australian Idol, singing Stevie Wonder's "Ribbon in the Sky". He progressed through the Idol series establishing a strong fanbase and praise from the judges, who often made reference to his afro hairstyle which became a focal point of his image. He was declared the winner on 19 November 2003, and gained a recording contract with BMG, which later merged with Sony. Following his Idol win Sebastian was required to record his debut album in six days, but despite this time restriction the album included three songs co-written by him. He then travelled to the UK to compete in World Idol. Although he impressed the judges, with several suggesting he could win the competition, he finished seventh.
Sebastian's winner's single "Angels Brought Me Here" debuted at number one and was the highest selling single in Australia in 2003, reaching 4× platinum certification. In 2010 it was named the highest selling song of the previous decade, ahead of Anthony Callea's "The Prayer". "Angels Brought Me Here" held the record for being the highest selling single ever released by an Australian act until 2011. Sebastian's debut album Just As I Am was released in December 2003 and also achieved number one. Its first week sales of 163,711 units are the second highest one week sales in Australian chart history, the highest for an Australian act. It eventually reached sales just short of 7× platinum, and is the highest selling album ever released by an Australian Idol contestant. "Angels Brought Me Here" also reached number one in Malaysia, Singapore, the Philippines, Indonesia and New Zealand. Just As I Am reached number three and double platinum certification in New Zealand, with "Angels Brought Me Here" achieving platinum certification there. The second single "All I Need Is You" reached number one and platinum in Australia, and peaked at number five in New Zealand. In early 2004 Sebastian appeared at the Asian MTV Awards, was a guest judge on New Zealand Idol and performed on Indonesian Idol. After a promotional trip to Malaysia in April he travelled to Europe and the US to write for his next album, performing on American Idol while in the US.
Beautiful Life, Sebastian's second album, had a more R&B edge to it and included songs he co-wrote with Robin Thicke and Brian McKnight and "Forever with You", a duet with American R&B singer Mýa. The lead single "Out with My Baby" debuted at number one in October 2004, and gained platinum certification. Sebastian performed at the 2004 ARIA Music Awards where he received the award for the Highest Selling Single for "Angels Brought Me Here", and the Channel V Oz Artist of the Year Award. He was also nominated for Highest Selling Album for Just As I Am. Beautiful Life peaked at number two and was also certified platinum. Sebastian embarked on a national tour in November, with a second stage from March to June 2005. Two further singles were released from the album, "Kryptonite" which peaked at number fifteen, and "Oh Oh" at number eleven. In 2005 Sebastian received awards for Favourite Video, Favourite Music Artist, and Favourite Aussie at the Nickelodeon Kid's Choice Awards, and a MTV Video Music Award for "Out With My Baby". He also received a Highest Selling Album ARIA Award nomination for Beautiful Life, and was named as a state finalist in the Young Australian of the Year Awards.
Sebastian's third album Closer to the Sun was mainly co-written with Australian musicians, and was a mix of genres including pop, R&B, soul, pop-rock and jazz. The lead single "Taller, Stronger, Better" debuted at number three in August 2006, and achieved gold certification. The album peaked at number four and was certified platinum. The second single "Elevator Love" reached number 11 and gold certification. The video featured Jennifer Hawkins as his love interest. Sebastian received an award for most played Urban work for "Oh Oh" at the 2006 APRA Music Awards. "Oh Oh" was also awarded Best Video and Sebastian named Best Male Artist at the Urban Music Awards, and for the second year in succession he received the Fav Aussie award at the Nickelodeon Kids Choice Awards. Closer to the Sun's third single "Cover On My Heart" was released in August 2007, peaking at number thirty two on the Aria chart. Closer the the Sun was named Best Album at the 2007 Urban Music Awards and Most Popular Album at the Dolly Teen Choice Awards.