Authorities were alerted when a 30-year-old man and 26-year-old woman failed to claim their luggage. The couple was sharing a cabin and traveling with family and friends on a 10-day Pacific Island cruise.
Officials are conducting the search Thursday off Australia's east coast for the couple, identified only as Australian citizens from New South Wales state.
New South Wales Police Superintendent Mark Hutchings says surveillance camera footage showed that the couple fell from the ship's mid deck Wednesday night, when the ship was about 120 kilometers off the coast of Forster.
"This is a tragic event at the moment, but we're holding out hope we might be able to find these people alive," Hutchings told reporters.
This was the latest in a series of high-profile incidents involving the Carnival cruise line. Last year, the Costa Concordia ran aground off the coast of Italy, killing 32 people. Also last year, the Costa Allegra caught fire and lost power in the Indian Ocean, leaving passengers without working toilets, running water or air conditioning for three days. Costa is a division of Carnival Corp.
Then in February, passengers aboard the Carnival Triumph spent five days without power in the Gulf of Mexico after an engine-room fire disabled the vessel. Those on board complained of squalid conditions, including overflowing toilets and food shortages.
The Associated Press contributed to this report.