A 35-year-old previously well man presents to his GP with 3 days of fever (38.3 °C), productive cough and pleuritic right-sided chest pain. He is a non-smoker, has no chronic disease and takes no regular medications. Observations: BP 118/72 mmHg, HR 96 bpm, RR 20/min, SpO2 96% on room air. Chest examination reveals right basal crackles; chest X-ray shows right lower lobe consolidation. He has no penicillin allergy. According to Australian Therapeutic Guidelines, what is the most appropriate initial antibiotic therapy?