In June 2011, Sa'u re-signed with the Newcastle club for a further two years.[7]
Melbourne Storm
On 18 August 2012, Sa'u was released from the final year of his Newcastle contract and signed with the Melbourne Storm on a one-year contract starting in 2013.[8]
Salford Red Devils
In June 2013, it was announced that Sa'u had signed a two-year deal with Salford in the Super League starting in 2014.[9]
Scoring 12 tries in his début season with Salford, he was voted k'Supporter's Player of the Year' for 2014.
Wakefield Trinity
On 14 May 2019, after five years with the Salford club, Sau joined fellow Super League team Wakefield Trinity on an initial one month loan.
In 2013 Junior played for Samoa in the Pacific Rugby League International against Tonga. He was also called in the 2013 Rugby League World Cup squad to replace the initially selected Roy Asotasi.