Starc returned home earlier this month from Australia's one-day series in England with lower back soreness and CA's worst fears were realised with confirmation of his injury.
"Mitch Starc had scans following his arrival back to Australia and they have shown an early stage low back stress fracture," CA's chief medical officer Justin Paolini said in a statement.
"His management plan will be determined in the coming days but he is expected to be unavailable for a prolonged period."
CA had better news concerning another paceman Jackson Bird, who also returned home early from England with back trouble.
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"Jackson returned home with some lower back soreness. We have ruled out a major injury but will review scans in two weeks to confirm this," Paolini said.
"We expect that he will be available during The Ashes (in November)."
Starc joins other Australian fast bowlers James Pattinson and Pat Cummins on the sidelines with back injuries.