Knowles is a member of the Australia men's national field hockey team. In 2006, he represented Australia at the Azlan Shah tournament in Malaysia. He competed in the 2007 Champions Trophy competition for Australia. In December 2007, he was a member of the Kookaburras squad that competed in the Dutch Series in Canberra. In January 2008, he was a member of the senior national team that competed at the Five Nations men's hockey tournament in South Africa. He was a member of the 2009 Champions Trophy winning team, playing in the gold medal match against Germany that Australia won by a score of 5-3. New national team coach Ric Charlesworth named him, a returning member, and fourteen total new players who had few than 10 national team caps to the squad before in April 2009 in a bid to ready the team for the 2010 Commonwealth Games. He was a member of the national team in 2010. That year, he was a member of the team that finished first at the Hockey Champions Trophy. Because of other commitments, he could not compete at the Azlan Shah Cup in Malaysia in May 2011. In December 2011, he was named as one of twenty-eight players to be on the 2012 Summer OlympicsAustralian men's national training squad. This squad will be narrowed in June 2012. He trained with the team from 18 January to mid-March in Perth, Western Australia. In February during the training camp, he played in a four nations test series with the teams being the Kookaburras, Australia A Squad, the Netherlands and Argentina. He is one of several Queensland based players likely to play in a three game test series to be played in Cairns, Queensland from 22 to 25 June against the New Zealand's Black Stickss. Final Olympic section will occur several days before this test and his inclusion in the series will be contingent upon being selected.
In 2007, Knowles was honoured by being named the Young Player of the Year by the International Hockey Federation. In 2015, Knowles was honoured by being named the 2014 World Player of the Year, following on from his performances at the 2014 Hockey World Cup, by the International Hockey Federation.