The 59-year-old Englishman was taken to hospital on Monday after suffering abdominal pains but discharged after tests, with his employer Channel Nine reporting that he was in "good spirits".
He returned to the Melbourne Cricket Ground commentary box on Wednesday to commentate on day three of the test but was taken to hospital again after suffering the same symptoms, a Cricket Australia spokesman told reporters.
Nicholas played 377 first class games and led English county side Hampshire for over 10 years before retiring in 1995 to carve out a career in journalism and broadcasting.
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(Reporting by Ian Ransom in Melbourne; Editing by Nick Mulvenney)
