scout Darren Bell discovered Duffie playing rugby union and offered him a scholarship. He moved to Melbourne at the end of 2008 and developed rapidly in their professional program. Duffie was a part of the Storm's 2009 NYCGrand Final winning team. He scored a try on the wing in the victory against the Wests Tigers. Duffie made his first-team debut against at the Twickenham Stoop in the World City Challenge in 2010. He scored two tries in his NRL debut in round seven of the 2010 season. and ended the year with a tally of 8 from 14 games played. In the same year, he was named the Melbourne Storm Rookie of the Year. Duffie played a major part in the Storm's comeback in 2011 scoring 12 tries in his 18 games played. He underwent shoulder reconstruction surgery in October of that year. In April 2011, Duffie signed a three-year extension to his contract with the Storm, taking him to the end of the 2014 season. In 2012, he scored 10 tries in the 17 games he played. In the Storm's round 19 match against the North Queensland Cowboys, he suffered another shoulder dislocation injury, to the same shoulder as in 2011. This caused him to miss out on Melbourne's 2012 Grand Final win. After returning from his shoulder reconstruction, Duffie played the opening four games for the Storm during the 2013 season. He started the year scoring three tries, including a double against the Brisbane Broncos at Suncorp Stadium. Following this, Duffie tore the anterior cruciate ligament in his left knee and re-injured his right shoulder whilst playing for the Cronulla Sharks in round 5 of the New South Wales Cup competition. This injury ended his 2013 season. Duffie returned to the lineup in 2014 playing in Melbourne's opening trial against the Canberra Raiders. He went to New Zealand and played in the NRL Auckland Nines competition, where he played in all three of the Storm's games. On 19 February, Duffie collided with a teammate during a training session and injured the same knee from 2013, rupturing his left anterior cruciate ligament. This caused him to miss the entire 2014 season. In late September 2014, he signed a one-year extension contract to the Storm, seeing him through to the end of 2015.
Duffie has signed to play rugby union for the Blues and North Harbour in 2016. Duffie had become a regular starter for those teams by the end of 2016 after consistently performing for a high level. Duffie cemented his spot as a regular starter for the Blues in 2017, starting for the Blues against the touring British and Irish Lions side in the Blues' historic 22-16 victory over them. Duffie was subbed off in the 65th minute after colliding with the Lions' replacement back Liam Williams twice during the match. Williams was yellow-carded in the 56th minute for his second collision with Duffie. Following All Blacks vice-captain Ben Smith taking a sabbatical and season-ending injuries to All Black wingers Israel Dagg and Nehe Milner-Skudder and fullback Jordie Barrett, Duffie was called up to the All Blacks as an injury replacement and was named in the 37-man squad for the 2017 end-of-year tour. Duffie made his debut for New Zealand in the 31-22 win over the Barbarians on 4 November 2017, as a replacement off the bench for in-form winger Waisake Naholo in the 67th minute. Duffie made his first international start ten days later against a French XV, scoring his first international try in the 23rd minute. Duffie played the full match and the All Blacks went on to win 28-23, however this match does not count as an official test match.
