The 26-year old Australian centre-half has signed a two-year deal with the Roar after spending most of the past eight years in Europe with Serbian clubs.
Susak grew up in Sydney and played with Sydney Olympic until he moved to Serbia to join FK Vojvodina in 2002. During his time there he had loan spells with FK Veternik and CSK Pivara before moving back to Australia and linking up with Adelaide United for an A-League season.
In mid-2008 he signed with Unterhaching and now finds himself back in his homeland with a club who rate him highly.
"Susak give us some great options ... and if you look at the season, there are lot more games and we're going to need a strong squad," Postecoglou said.
"I think he'll be fighting for a first XI spot but also giving us a lot of depth in the right areas.
"I think he's the kind of guy who has a real desire to succeed and who'll do well in the Hyundai A-League."