Petersen has a viral infection and a decision on his fitness will be made on the morning of the match.
The 21-year-old De Kock, who was handed a central contract by Cricket South Africa on Tuesday, has not played test cricket but is a rising talent in the one-day international side.
Opening the batting, he scored centuries in three consecutive innings against India in December.
If left-handed batsman De Kock is selected he will likely take over as wicketkeeper from AB de Villiers.
Australia lead the three-test series 1-0.
(Reporting by Nick Said; editing by Toby Davis)
