Only following determined coaxing by friends, Leith auditioned for the 4th series of Network Ten's Australian Idol along with some 25,000 other contestants. He was selected by the judges as one of the final 24 contestants, and was the first person voted into the competition's top 12 as the result of a nationwide viewer vote. He was praised by the show's judges for his falsetto technique, which he used regularly in performances. For the first time, Australian Idol 2006 permitted contestants to perform with instruments, and Leith accompanied himself on guitar for his renditions of Chris Isaak's "Wicked Game", Leonard Cohen's "Hallelujah", Alex Lloyd's "Never Meant to Fail" and Ben Harper's "Waiting on an Angel", and on piano for his rendition of Split Enz's "Message to My Girl". Leith also played piano for the non-competition round "Up Close and Personal" night, when he performed his original song "Sky".