Fonua was born and raised in Ponsonby, Auckland, New Zealand to Tongan lawyer Sione Fonua and British-born mother Julie. He holds dual Tongan and New Zealand citizenship. His family includes two other sisters.
Fonua's swimming career began at the Roskill Swimming Club based at Cameron Pool in Auckland, coached by Sandra Burrow from 1999–2007. He broke numerous Auckland and New Zealand Age Group Records under Burrow's tenure. He then moved to West Auckland Aquatics in 2007, and was coached by Donna Bouzaid. In the Fall of 2008, Fonua enrolled at Texas A&M on a swimming scholarship. While at Texas A&M he was a peer voted Team Captain, Big XII Conference Champion, NCAA All-American, and recipient of The Aggie Heart Award. He graduated with a Telecommunication and Multi-Media degree, with a Minor in Creative Writing in May 2013.
In preparation for the 2012 London Olympics Fonua was trained by New Zealander and designated head coach for Tonga, Jon Winter. He served as his nation's flag-bearer in the 2012 Summer Olympics Parade of Nations. As a swimmer at the 2012 Summer Olympics, he competed in the Men's 100 metre breaststroke, failing to reach the semifinals.
Fonua made an international comeback at the 2015 Pacific Games in Port Moresby, Papua New Guinea. He created history by becoming the first ever Tongan athlete to ever win 3 Gold medals at a Pacific Games by sweeping the Breaststroke events, setting 2 Games Records in the process (50 m and 100 m Breaststroke). He is the only Tongan athlete in history to ever hold dual Oceania and Pacific Games titles.