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.
"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.