On 23 January 2015, Walker was named in the South Sydney Rabbitohs tournament winning 2015 NRL Auckland Nines squad. On 13 February, Walker played centre for NRL All Stars for the 2015 All Stars match at Cbus Super Stadium. Walker scored a try in the side’s 20-6 loss to the Indigenous All Stars. On 23 February 2015, Walker played for the Rabbitohs in the 2015 World Club Series match against 2014 Super League Grand Final premiers St. Helens, playing at centre and scored a try in the Rabbitohs 39-0 win at Langtree Park. In the 2015 NRL season Round 1 opener against the Brisbane Broncos at Suncorp Stadium, Walker scored the first try of the 2015 season in the Rabbitohs 36-6 win. On 13 June 2015, Walker was included into the New South Wales Blues origin squad as 19th man leading up to Game 2 of the 2015 State of Origin series. Walker finished the 2015 NRL season with him playing in 22 matches and scoring 13 tries for the Rabbitohs. In the early hours of 22 September 2015, Walker and fellow South Sydney teammate and best friend Aaron Gray were hospitalized after the pair overdosed on prescription drugs Oxycodone and Tramadol at Gray's apartment in Rosebery. The pair were discovered by another friend of them who alerted emergency services and the pair were sent to St Vincent's Hospital in a critical condition placed in induced comas. The pair were released from hospital three days later to front the media about the unfortunate situation. Later on 26 November, Walker was released from the final year of his Rabbitohs contract after requesting a release, after finding out that he was fined five percent of his contract and the club angry at him for bringing the club into disrepute.