In 2004, Demetriou joined the Wakefield Trinity Wildcats where he spent the next seven seasons, captaining the club for five of those seasons. In doing so, he became the first Australian player without NRL experience to captain a Super League team. In 2006, he was named Wakefield Trinity Wildcat's Player of the Year. That season he scored a late try against local rivals, the Castleford Tigers, to keep Wakefield Trinity Wildcats in the Super League, simultaneously relegating Castleford Tigers. The moment was captured and a large copy of the photo is on display outside the coach's office at Belle Vue. In 2007, he was named as Wakefield Trinity Wildcats's Player of the Year again. He was also named at centre in the Super League Dream Team, and was short-listed for the Man of Steel award along with Trent Barrett and James Roby. In 2009, he wore the number 8 jersey for Wakefield Trinity Wildcats, in memory of his close friendAdam Watene. Demetriou is associated with the Adam Watene Fund, helping raise funds for his family in New Zealand. In 2010, he was awarded a 3-month testimonial at the Wakefield Trinity Wildcats as a reward for his hard work and dedication to English rugby league.
Coaching career
Demetriou's contract with Wakefield Trinity Wildcats expired at the end of 2010, and he was told that it would not be renewed. He initially joined the York City Knights for 2011, but a subsequent offer as player-coach for Keighley Cougars arose, and was accepted. In his first season in charge, Keighley gained promotion to the Kingstone Press Championship. Demetriou returned to Australia in 2013, becoming the head coach for the Northern Pride in the Queensland Cup. During his two seasons at the Pride, the club won back-to-back minor premierships, the 2014 Queensland CupGrand Final and the inaugural NRL State Championship game. In 2014, he coached the Queensland Residents side, who defeated the NSW Cup rep side 24–16. In October 2014, Demetriou joined the North Queensland Cowboys as an assistant coach. On 4 October 2015, Demetriou was a member of the Cowboys' coaching staff in the side's 17–16 Grand Final victory over the Brisbane Broncos. In January 2016, Demetriou joined the St George Illawarra Dragons as an assistant coach and as head coach to their NSW Cup feeder team, the Illawarra Cutters. In September 2016, he was named coach of the 2016 Intrust Super Premiership NSWTeam of the Year. In 2017, Demetriou joined the Brisbane Broncos under head coach Wayne Bennett. He took over as the side's attacking coach in Round 6 of the 2018 NRL season, with the side scoring the second most tries in the competition from that point forward. In December 2018, he joined the South Sydney Rabbitohs as an assistant coach, following Bennett to the club. In February 2020, it was announced that Demetriou would succeed Bennett as head coach at the end of the 2021 NRL season.