After playing lower grades with the South Sydney Rabbitohs in 2011 and Parramatta Eels in 2012, Fonua-Blake joined the St. George Illawarra Dragons in 2013. In 2013 and 2014, he played for the Dragons' NYC team. In 2014, he co-captained the side. On 2 September 2014, he was named on the interchange bench in the 2014 NYC Team of the Year. On 18 October 2014, he played for the Junior Kiwis against the Junior Kangaroos, before re-signing with the Dragons on a contract to the end of 2016. On 19 January 2015, he was stood down by the Dragons due to disciplinary reasons, before having his contract terminated on 30 June 2015 when he pleaded guilty to assaulting his girlfriend. At the end of 2015, he was granted approval by the NRL to train with the Manly Warringah Sea Eagles from 1 November of the same year.
2016
After playing in five Intrust Super Premiership NSW matches for the Sea Eagles, Fonua-Blake was cleared to be eligible to play in the NRL after satisfying the NRL Integrity Unit that he had completed a comprehensive off-field counselling and development program. This allowed him to make his NRL debut for the Sea Eagles against the Parramatta Eels in Round 7 of the 2016 NRL season. On 7 September, following the end to Manly's season, Fonua-Blake was named in the New Zealand Kiwis train-on squad for the 2016 Four Nations tournament. Later on the same day, he won the Ken Arthurson Award as Manly's 2016 Rookie of the Year. He played in 14 games for the Sea Eagles in 2016, crossing for 2 tries.
2017
Fonua-Blake made 22 appearances for Manly in 2017 as the club finished 6th on the table and qualified for the finals. Fonua-Blake played in the elimination final against the Penrith Panthers which Penrith controversially won 22-12 as Tyrone Peachey scored the match winning try although replays had shown the player knocked the ball forwards with his hands.
2018
Fonua-Blake made 24 appearances for Manly in 2018. The club endured a horror season on and off the field narrowly avoiding the club's first wooden spoon by 2 competition points.
2019
Fonua-Blake made 22 appearances for Manly in the 2019 NRL season as the club finished sixth on the table. Fonua-Blake scored a try in Manly's week one elimination final victory over Cronulla at Brookvale Oval. Fonua-Blake played in Manly's elimination final defeat against South Sydney at ANZ Stadium the following week which ended their season.
2020
In round 8 of the 2020 NRL season, Fonua-Blake was sent off in the final seconds of the match against Newcastle after he called the referee a "Fucking retard", and later spastic. Fonua-Blake said this in response to Manly being denied a penalty try in the final moments of the match and saw Newcastle hold on for a 14-12 win at Brookvale Oval.
Assault charge
In 2015 he pleaded guilty to assault charges after pushing and kicking his partner after a night out drinking. He was fined $1000, ordered to undergo counselling and given a one year suspended sentence. His contract with the Dragons was also terminated.
