Susak, who has spent the past two seasons playing in the German third division with SpVgg Unterhaching, will give the Roar some versatility in defence, according to coach Ange Postecoglou.
"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."
Having snapped up the 26-year-old, Postecoglou is now looking to add a striker to his squad.
"We have one visa spot left and that's where we are concentrating," Postecoglou said.
"We have a few options but I'm not going to rush into that one, I want to get it right.
"We need depth all over the park ... with such a condensed calendar and injuries and fatigue, I want us to have a strong squad."