Paul Rossington and Kristen Schroder, from New South Wales state, 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 Police Superintendent Mark Hutchings.
Surveillance camera footage showed the couple fell from the ship's mid deck Wednesday night, Hutchings said, when the ship was about 120 kilometers (65 nautical miles) off the coast of Forster, a city 300 kilometers (185 miles) north of Sydney.
Rescue officials searched through the night, but had not found the couple as of Friday morning, police said.
"We're going to be going hard today -- we've got a lot of assets we're throwing at this," Hutchings told Australian Broadcasting Corp. on Friday.
Investigators were having the surveillance video enhanced in a bid to determine whether Rossington and Schroder had jumped or had fallen by accident.
The ship has around 600 surveillance cameras that are constantly monitored, although no one reported seeing the fall at the time.
No life preservers were missing from the ship, Hutchings 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 and seven of their family and friends were among 2,680 passengers on a South Pacific cruise. The ship's last stop was Mare Island in New Caledonia, which it left on Monday, bound for Sydney. The couple 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 that they were alerted of the missing passengers 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.
Carnival Corp.'s representative in the South Pacific region, Ann Sherry, is chief executive of Carnival Australia, which represents 80 percent of cruise lines in the region. She told an Australian parliamentary inquiry into cruise ship safety in February that only two passengers had disappeared overboard from cruise ships in the South Pacific since she took her job more than five years earlier. She said both disappearances had been fully investigated by authorities and there had been no suspicious circumstances in either case.
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