Steyn is set to spend at least six months on the sidelines after breaking his shoulder while bowling during the Proteas 177-run win in the first Test in Perth, which finished Monday.
Pretorius, who made his one-day debut earlier this year, will join the squad in Hobart for the second Test starting on Saturday, Cricket South Africa said on its website.
Morne Morkel is seen as the leading contender to replace Steyn in the starting team for Hobart. The third and final Test is in Adelaide.
AFPUncapped all-rounder Dwaine Pretorius has been called into the Proteas Test squad for the series against Australia as cover for Dale Steyn.
Original source: Proteas call up Pretorius for injured Steyn