A search is underway along a large stretch of the New South Wales coast for two people who are missing from a cruise ship.

The man and the woman, believed to be a couple, were reported missing when a Carnival Cruise Liner docked at Circular Quay in Sydney this morning.

Westpac Lifesaver Rescue Helicopter chief executive Stephen Leahy says it is believed the pair fell overboard.

He says the search stretches from Sydney Harbour to Newcastle.

"We've been advised that a ship docked at Circular Quay this morning at 5:30 and at that stage two persons were found missing," Mr Leahy said.

"An extensive search from Sydney Harbour all the way up the coast following the track of that cruise ship is now underway."

A spokesman for Carnival Cruise Lines says a 27-year old woman and a 30 year old man were booked on a 10-day cruise and could not be accounted for during the disembarkation process.

He says Carnival is co-operating with police and out of respect for families of the missing passengers will not be making any comment on the matter.

Detective Superintendant Mark Hutchings, from the NSW Police Marine Area Command, says there is a chance the pair may still be alive.

"It's game on and we are going to be pulling out all stops to find these people," he said.