A desperate air and sea search has been launched for a cruise ship passenger feared to have fallen overboard.

Two cruise ships - the Carnival Splendor and P&O's Pacific Adventure - were due to dock in Sydney Harbour before 6am on Monday.

The ships have been forced to remain offshore as the search continues.

A man on the Pacific Adventure told 2GB radio that he and 2,600 other passengers woke to a 'man overboard call' early on Monday. 

'We got woken up with the man overboard call and from there it's been all on for a couple of hours now,' he said. 

A driver who was supposed to pick up passengers from the Pacific Adventure explained that he was told not to rush because the 'the ship's stopped outside the head and somebody's gone overboard.' 

P&O Cruises confirmed a search is underway and that staff were providing the missing passenger's family with support. 

'P&O's Pacific Adventure is currently undertaking a search and rescue operation off Sydney after a person overboard alarm was raised at 4am,' a spokesperson said. 

A desperate search has been launched for a cruise ship passenger feared to have fallen overboard during the ship's return to Sydney (stock image)

The Carnival Splendor and P&O's Pacific Adventure were due to dock in Sydney Harbour before 6am on Monday (both ships are pictured in waters off Sydney's eastern suburbs)

'Since this time, the ship carrying more than 2,600 guests, has been onsite searching along with other nearby vessels deployed to the area by the Australian Maritime Safety Authority's Joint Rescue Coordination Centre.

'The family of this guest is being cared for by our onboard team while every effort is being made to find them.' 

Pacific Adventure's arrival into Sydney Harbour has been cancelled as the large-scale search, which includes a rescue helicopter, continues. 

A NSW Police spokesman confirmed an urgent search was underway following reports a passenger had disappeared 10 nautical miles from Sydney Heads. 

'Police are leading the search of waters off Sydney following reports a person was in the water,' the spokesman said. 

'NSW Police Marine Area Command have deployed a number of resources to search for the person and are leading the co-ordination of the water search.'

Passengers on the Pacific Adventure woke to a 4am 'man overboard alarm' on Monday (pictured is vessels in the search zone near Sydney Heads)

Pacific Adventure's arrival into Sydney Harbour has been cancelled as the large-scale search, which includes a rescue helicopter, continues (stock image)

The Pacific Adventure is returning from a three-day cruise to Queensland's Morton Island which is understood to have had an Elvis theme. 

Carnival Splendor also remains at sea to ensure search efforts are not hindered. 

More to come.