(CNN) A cruise ship spilled thousands of gallons of liquid food waste in the Great Barrier Reef in August, according to an ongoing investigation by the Australian Maritime Safety Authority -- potentially damaging a reef already plagued by pollution and coral bleaching.

Carnival, which owns the P&O cruise ship, said the spill was unintentional and amounted to 7,000 liters (1,849 gallons). But Green Party Senator Larissa Waters and local media report the spill was 27,000 liters (7,133 gallons), citing an Australian Maritime Safety Authority report presented to the Senate. That amount is equivalent to more than 100 bathtubs full of waste water.

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The P&O ship spilled the polluted water on August 26, but authorities were not notified until two days later when Carnival self-reported the incident.

Australian authorities detained the ship when it returned to Sydney in early September, according to an Australian Maritime Safety Authority representative. Carnival had to pay a fine of about $1.5 million (2.1 million Australian dollars) to release the vessel.

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