The team also contains familiar faces in veteran Cahill, captain Mile Jedinak and 10 others who featured at the World Cup.
The September friendlies, to be played in Liege and London, will build on the Socceroos' World Cup progress and help prepare them for the Asian Cup to be held in Australia in January.
"The next series of games for us are really important to broaden the base and depth so we're going to be pretty bold in the way we select our squads and give some people an opportunity," Postecoglou said.
"After the World Cup campaign we had a break and I had a bit of a chance to think about everything that transpired over the last six to eight months, culminating in the World Cup.
"One thing that came out, that was evident to me, is that we need to broaden our base and get some depth in various positions in our squad."
Postecoglou said for the Socceroos to have a strong squad at the Asian Cup, they had to "expose some young players to international football".
Smith, who made his English Premier League debut last year for Liverpool, was one such player, the coach said.
"Brad's a player we have been monitoring since I've been in the job and he plays in a position where we don't have a great deal of depth. He plays left-back and left side and he's quick," Postecoglou said.
Experienced campaigners Mark Bresciano and Josh Kennedy were left out but could still be included for further friendlies in October and November, Postecoglou added.
The Socceroos play World Cup quarter-finalists Belgium in Liege on September 4 before facing Saudi Arabia in London on September 8.
Goalkeepers: Mat Ryan (Club Brugge/BEL), Mitch Langerak (Borussia Dortmund/GER)
Midfielders: Mile Jedinak (Crystal Palace/ENG), Mark Milligan (Melbourne Victory/AUS), Massimo Luongo (Swindon Town/ENG), Terry Antonis (Sydney FC/AUS), Joshua Brillante (ACF Fiorentina/ITA), Chris Herd (Aston Villa/ENG), Carl Valeri (Melbourne Victory/AUS)
Forwards: Tim Cahill (New York Red Bulls/USA), Tommy Oar (FC Utrecht/NED), Ben Halloran (Fortuna Dusseldorf/GER), Matthew Leckie (FC Ingolstadt/GER), Tomi Juric (Western Sydney Wanderers/AUS), Robbie Kruse (Bayer Leverkusen/GER)
- Sports & Recreation
- Tim Cahill
- Chris Herd
- Mile Jedinak
- Trent Sainsbury
- Bailey Wright
- Ange Postecoglou