Fonua played for the Storm's NYC team from 2010 to 2012, scoring 32 tries in 45 games. At the end of the 2011 season, Fonua was named at in the 2011 NYC Team of the Year.
2012
In Round 23 of the 2012 NRL season, Fonua made his NRL debut for the Storm against the Gold Coast Titanson the wing in the Storms 24-16 win at AAMI Park, becoming the first Victorian born and bred player to represent the Storm in first-grade. In Fonua's third game for the Storm in the Finals 1 week match against the South Sydney Rabbitohs, Fonua scored his first NRL try in the 24-10 win at AAMI Park, after previously being denied tries on four occasions by the video referee, once in his debut, twice in his second game, and once prior to his try in his third game. Fonua finished off his debut year with 4 matches and one a try. On 2 October, Fonua was named at in the Junior Kangaroos team to face the Junior Kiwis.
2013
Fonua played in Storm's 2013 World Club Challenge victory over Leeds Rhinos. He was selected to play for his parents' country of origin Tonga on 20 April against Samoa in the Pacific Rugby League International at Penrith Stadium. He scored 2 tries in the 36-4 victory. On 10 June, Fonua extended his contract with the Storm, keeping at the club till the end of the 2015 season. In Round 21 against Raiders at Canberra Stadium, Fonua scored a hat-trick of tries in the 68-4 win including a spectacular set up for Will Chambers after the ball seemed almost certain to go dead, with some commentators comparing it to the famous set up by Greg Inglis during the centenary test. Fonua was selected in the Tonga 24 man 2013 Rugby League World Cup squad, playing in 2 of their 3 matches.
2014
Fonua was selected in the Storms' inaugural 2014 NRL Auckland Nines squad. He finished the Storms' 2014 NRL season with him playing in 20 matches and scoring 10 tries.
On 1 August, he signed a 3-year contract with Super League team Hull F.C. starting in 2016. Fonua had an outstanding first season with the club and quickly established himself as a fan favourite. He helped Hull F.C. reach the Challenge Cup Final where they would go on to beat the Warrington Wolves by a narrow score of 12-10. Fonua scored the first of Hull FC's two tries at Wembley Stadium. In his first season in the Super League, he scored 15 tries which contributed to him being selected for the Super League Dream Team along with 5 other Hull teammates.
2017
On 9 June 2017, it was confirmed that Fonua would return to the NRL, to join the Wests Tigers, starting in the 2018 season. On 26 August 2017, Fonua would go on to win the Challenge Cup for a second year in a row, contributing massively to the 18-14 win over the Wigan Warriors by scoring two of Hull's three tries at Wembley Stadium. Fonua finished the year with making the 2017 Super League Dream Team for a second year in a row, but this time as a winger.
2018
At the start of the 2018 season, Fonua spent the first few weeks playing for The Western Suburbs Magpies in The Intrust Super Premiership NSW. After a number of good performances, Fonua was called up to play for The Wests Tigers. Fonua made his debut for the club in their 22-20 defeat by The Parramatta Eels. He played two games on the wing before being switched to centre. On 15 May, Fonua was demoted to reserve grade for showing up late to training, days after a Man of the Match performance. Weeks later, he was sidelined after breaking his arm, but made a return later in the year. Despite only scoring one try from his ten appearances, Fonua lead the club and was fifth in the NRL for post-contact metres. He also gained attention for his mullet haircut. For the 2020 season after a season ending injury Fonua announced his return to Super League side Hull FC
Honours
Club
Melbourne Storm
World Club Challenge: 2013
Hull F.C.
Challenge Cup: 2016, 2017
Individual
Super League Dream Team: 2016 as a Centre and 2017 as a Winger