The Great Barrier Reef Marine Park Authority and Australian Institute of Marine Science say the current , living reef started growing about 20,000 years ago . The Great Barrier Reef Marine Park Authority ( GBRMPA ) considers the earliest evidence of complete reef structures to have been 600,000 years ago . According to the GBRMPA , the current , living reef structure is believed to have begun growing on the older platform about 20,000 years ago .