The 30-year-old becomes the second Rams player this summer to move to the A-League side after defender Andy Todd made the switch last month.
Derby boss Nigel Clough told the club's official website: "It is an important year for Mile if he wishes to be involved in Australia's World Cup squad, and for that reason alone he needs to be playing regular first-team football.
"It is not something I could guarantee here so it is in the best interest of both parties for him to return to Australia. Mile is an excellent pro and I have no doubt he will be a big star in the A-League.
"He leaves with the best wishes of everybody at Derby County."
Sterjovski made 30 appearances for the Rams and scored two goals after joining the club from Turkish side Genclerbirligi in January 2008.