The 23-year-old was a replacement in the opening Test wins over Romania and the Pacific Islanders.
Callam replaces Johnny Beattie, who suffered ankle ligament damage in the win over the south seas team last weekend.
Glasgow Warriors stand-off Dan Parks keeps his place in the starting XV against the Wallabies, while Mike Blair replaces Chris Cusiter, who has undergone shoulder surgery.
Scotland head coach Frank Hadden has called on his team to keep their undefeated home record in 2006.
He said: "We are unbeaten at Murrayfield this year and everyone involved with the squad is proud of that.
"We will require a top-drawer performance from our entire 22 to maintain that on Saturday."
Rory Lawson and Alasdair Strokosch start on the bench as they join a senior Scotland squad for the first time.