'Person overboard' alarm rings out on ship before dawn

"NSW Police Marine Area Command have deployed a number of resources to search for the person and are leading the coordination of the water search," they said in a statement.

Cruise operator P&O said a "person overboard alarm" was raised on the ship at 4am.

The vessel, with more than 2,600 guests onboard, is involved in the search.

"The family of this guest is being cared for by our onboard team while every effort is being made to find them," the spokesperson said.

Other vessels were deployed by the Australian Maritime Safety Authority's Joint Rescue Coordination Centre.

The Pacific Adventure was due to dock at Sydney's White Bay Terminal cruise terminal early on Monday morning.