Fainga'a made his Super 14 debut in 2006. He was fast-tracked into the Brumbies’ side due to injuriues in the squad, and played his first senior match for the Brumbies against the Stormers in Cape Town. Later that year he captained the Australian Under 19s team to win the IRB World Championship in Dubai. Fainga'a went on to earn 27 super rugby caps at the Brumbies from 2006 to 2008. In 2008, Saia and his twin brother Anthony signed on to play with the Queensland Reds for the 2009 season. He made his Test debut for Australia in 2010 against Fiji in Canberra. He played in all 18 matches of the Reds 2011 Super Rugby title-winning season, and was selected alongside his brother Anthony in the Wallabies squad for the 2011 Rugby World Cup in New Zealand, where Australia took third place.
^ abcdSen, Djuro (2010), "By Hook or by Crook", Official Program, Qantas Wallabies Matchday, Australia v Ireland – Saturday, June 26, 2010, Australian Rugby Union, pp. 14–17, archived from the original on 21 February 2014, retrieved 24 June 2010