The couple from New South Wales were discovered missing Thursday morning after the Carnival Spirit docked at Sydney's Circular Quay, at the end of a 10-day journey, said New South Wales state Police Superintendent Mark Hutchings.
He said surveillance camera footage showed that the couple - a 30-year-old man and a 27-year-old woman - fell from the ship's mid deck Wednesday night, when the ship was about 120 kilometers (65 nautical miles) off the coast of Forster, a city 300 kilometers (185 miles) north of Sydney.
"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.
Investigators were having the video enhanced in a bid to determine whether the couple had jumped or had fallen by accident, Hutchings said.
No life preservers were missing from the ship, he said. A missing life preserver might have indicated that one of the missing passengers had attempted a rescue.
Jo Meehan, spokeswoman for the Australian Maritime Safety Authority, which is coordinating the search, said an airplane, a helicopter and police boats were searching a 1,000-square-kilometer (300-square-nautical mile) area of sea.
The couple had been among 2,680 passengers on a South Pacific cruise. They were discovered missing as passengers disembarked, said Peter Taylor, spokesman for the ship's operator, Carnival Cruise Lines.
"The guests in question were traveling with family and friends, and initial reports indicate that the couple was last seen onboard the vessel last night," Taylor said in a statement Thursday.
"The ship immediately initiated standard missing person procedures, including a full search of the vessel, as per protocol," he said.
Police said in a statement there were alerted about two hours after the ship docked.
Carnival Cruise Lines is a subsidiary of Miami-based Carnival Corp., the world's largest cruise operator.
Carnival Corp. has been plagued by a series of high-profile problems in recent years. 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.
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.