Matagi was first selected to play for Auckland in 2009. He was selected for the New Zealand Residents in 2009, 2010 and 2011. In 2011 Matagi was named in the Auckland Rugby League team of the year and went on to be named man of the match in the National Competition final. Matagi made his debut for the Auckland Vulcans in the NSW Cup on 3 April 2010 but could only play home games for the Vulcans due to travel restrictions.
He was named the 2012 New Zealand domestic player of the year. In early 2013 Matagi joined the New Zealand Warriors squad on a pre-season trial contract.
On September 2014, Matagi was selected for the New Zealand and Samoa Four Nations train-on squads. On 7 October 2014, Matagi was selected in the New Zealand Kiwis team final 24 man squad for the 2014 Four Nations series.