Barbarouses played 21 matches during his first stint with Wellington Phoenix in three seasons and scored two goals. The Phoenix club officially signed Barbarouses as one of its foundation players on 4 May 2007. Seventeen years old and still at school, he started as the club’s youngest player. He made his professional debut on 21 October 2007 against Central Coast Mariners, coming on as a substitute in the 86th minute. He played his first full professional game when he fronted against Central Coast Mariners on 19 January 2008.

As a member of the national Under-20 side he played an integral part in the 2008 OFC U-20 Championship qualification tournament for the 2009 Under-20 World Cup, scoring two goals in the three pool matches. New Zealand did not qualify for the final tournament.

Barbarouses scored his first professional goal on 18 January 2009 – an equaliser against Adelaide United. He scored his second A-League goal against Sydney FC on 1 November 2009 after coming off the bench to replace Leo Bertos. He scored the goal off a volley which bounced over Clint Bolton. Barbarouses played his first match starting in the first team line-up against Newcastle Jets on 4 November 2009, assisting Chris Greenacre’s goal with a back-heel pass.

Barbarouses turned down a two-year contract extension with Wellington Phoenix in February 2010 for a three-year contract with A-League side Brisbane Roar. Brisbane Roar play an attractive brand of football and Barbarouses wanted the opportunity to play more regular football. In the 2009–10 season, which took the Phoenix into the finals, Barbarouses had had only three starts, and ten appearances off the bench.

Barbarouses made his debut for the Roar in Round 1 of the 2010–11 season, starting against Gold Coast United in a 0–0 draw on 8 August 2010. Barbarouses continued to start for the Brisbane team and played against his old club Wellington Phoenix, on 27 August 2010 at Suncorp Stadium. In the 73rd minute, he received a through-ball from team-mate Matt McKay and slotted home his first A-League goal for the Brisbane Roar. Barbarouses was named in the 2010–11 A-League All-Star Team after 12 goals in 33 appearances.

Barbarouses made his senior debut in a 2010 FIFA World Cup qualifier against Fiji on 19 November 2008. He trained with the national team before its World Cup qualifying play-off against Bahrain. Barbarouses attended a 12-day All Whites training camp in Auckland in April–May 2010 as a member of a 15-man squad of Australasian-based players. He was one of two forwards on the squad.

Barbarouses had stated that he would like to make the New Zealand 2010 FIFA World Cup squad. Although widely tipped to make the final 23-man squad travel to the 2010 FIFA World Cup, Barbarouses’ name was missing when NZFA chairman Frank van Hattum read out the names at Auckland’s Sky City on 10 May 2010. He was, however, named as one of seven non-travelling reserves.

On 18 July 2011, Fairfax News NZ announced that Barbarouses had signed a three-year contract with Russian side Alania Vladikavkaz. Brisbane Roar received $600,000 as a transfer fee. Barbarouses made his debut for Vladikavkaz on 9 August 2011, in a First Division league match against Torpedo Vladimir. He played the full 90 minutes and scored the sole goal of the match, allowing Alania to win 1–0.

A standout first season with then newly crowned A-League champions Brisbane Roar saw the 21-year-old striker included in the New Zealand squad for a friendly against China in Wuhan on 25 March 2011.

On 27 June 2012, Barbarouses signed a one-year loan contract with Greek giants Panathinaikos. Barbarouses only played 11 games and scored no goals during his one-year loan spell in his ancestral country. His stand out moment was when he played against Greatest Rivals Olympiacos in a league match in Piraeus in a 1–1 draw.

Barbarouses formed part of an 18-man All Whites squad that played Jamaica in an international friendly at Mt Smart Stadium in Auckland, New Zealand on 29 February 2012.

On 23 May 2012, Barbarouses scored his first goal for the All Whites in a friendly match against El Salvador. His 64th-minute half-volley from close range levelled the scores and the match resulted in a 2–2 draw.

On 7 August 2013, Kosta signed a 3-year deal at Melbourne Victory under Ange Postecoglou for the second time. The first time was at rival team Brisbane Roar, following a successful career start at home town team Wellington Phoenix. Kosta debuted for Melbourne Victory in round 2 of the 2013–14 season in the 2–2 away draw with Adelaide United.

On 8 March 2016 it was announced that Barbarouses would leave Melbourne Victory at the end of the season to rejoin his hometown club Wellington Phoenix on a 3-year contract. On 5 June 2017 Phoenix announced they would be releasing Barbarouses following his request to leave for non-footballing reasons.

Following his release from Wellington Phoenix, Barbarouses rejoined former club Melbourne Victory on a two-year deal in June 2017. In June 2019 Melbourne Victory announced that his contract ended and he declined an extension.

Barbarouses was a member of New Zealands Qualification campaign for the 2018 World Cup in Russia. He played in the both the home and away matches vs Peru in November 2017 as New Zealand ultimately were unsuccessful in their qualification bid.

On 7 June 2019, Barbarouses signed on a multi-year contract deal with Sydney FC.