The 26-year-old attacking midfielder had only joined newly promoted Nagasaki in January, moving from German 2.Bundesliga club Heidenheim.
The former Brisbane Roar and Gold Coast United man only made five appearances for the Japanese club during his six-month stint.
Cairns-born Halloran has six Socceroos caps to his name including three at the 2014 World Cup in Brazil.
Several A-League clubs will be interested in Halloran who spent almost five decent years in Germany's 2.Bundesliga with Heidenheim and Fortuna Dusseldorf.