SYDNEY (AFP) - A massive air and sea search was under way Friday for an couple who fell overboard during a luxury cruise off Australia, as the cruise ship company involved said safety was its first priority.

Police said surveillance footage shows Paul Rossington, 30, and Kristen Schroder, 27, going overboard just before 9:00pm (7pm in Singapore) on Wednesday when the boat was about 60 nautical miles off Australia's New South Wales coast.

The pair, who are both Australian, were not discovered missing until the Carnival Spirit, which had completed a 10-day Pacific Islands journey, docked at Sydney's Circular Quay the following morning and the couple failed to disembark.

Police Marine Area Commander Mark Hutchings would not speculate on whether they jumped or fell.

"It's very unclear (the footage), but it does depict one person going over and then a short time (later) another person going over," he said.

"We can't tell from the footage which is which."

Searchers aided by aircraft from the Australian Maritime Safety Authority and a Navy helicopter are scouring an area which stretches from Sydney Harbour to Newcastle - north of Sydney - for Schroder and Rossington, who is a paramedic.

Hutchings said searchers were covering a 500 square nautical mile area and were hopeful the pair could be found alive, but admitted "it's getting more and more difficult as time goes on".

Carnival Australia chief executive Ann Sherry said safety and security were the number one priority for the cruise industry and such incidents were rare.

"The suggestion that this happens often is simply not true," she told reporters.

Sherry said the heights of railings on ships were regulated, and on this particular vessel they were higher than required by international protocols.

Asked whether it was possible that somebody could fall and trip overboard, Sherry said: "I think it would be highly unlikely, but again in this case the police are conducting a full investigation so I will wait and see what they come back with.

"The police have got the CCTV footage, they will determine and make a finding on exactly what's gone on," she said.

Police cleared the ship to leave port late Thursday, with the missing couple's room locked to future guests until further notice.