Cruise operator Carnival Australia said that a thorough search of the Sun Princess had failed to find the 84-year-old man, who was reported missing in the early hours of Tuesday.

The Sun Princess, which was due to dock at Sydney's White Bay terminal at 7am, has been turned around and is now retracing its path. It is now 195 nautical miles east of Sydney.

The Rescue Co-ordination Centre in Canberra is also involved in the search effort.

"Sun Princess' arrival in Sydney will be delayed and the next cruise will be impacted," a Carnival Australia statement said.

"We are contacting guests to advise them of this change."

The Sun Princess has been on a 13-day New Zealand cruise.

A passenger on board the boat, identified only as Barry, told 2UE radio that an announcement was made on the ship about 6.30am.

"We got an announcement from the captain ... and we've headed back out to sea," he told 2UE. "It seems that we have lost a passenger overboard."

He said the captain told passengers they had carried out a thorough search and found no trace of the man and that a rescue boat and plane were now on the way to aid the search effort.

"We are headed back out and he doesn't know when we are going to dock," Barry said.

More to come