US Coast Guard to resume search for missing Aussie

There are grave fears for an Australian man missing from a cruise ship en route to Hawaii, as the United States Coast Guard prepares to resume a search at first light. 

The passenger went overboard on the Quantum of the Seas in the Pacific Ocean about 800km south of Kailua-Kona, on the west coast of O'ahu also known as Big Island.

The ship departed Brisbane on April 12 for a two-week cruise to the Hawaiian Islands before tragedy struck. 

Joint Rescue Coordination Centre Honolulu received a report from the ship about a man going overboard at 11.03pm on Tuesday (Wednesday night AEST).

"The crew of the cruise ship remained on scene to search for approximately two hours, deploying six life rings," the coast guard said on Thursday.

A Hercules aircrew arrived in the search area at approximately 9am local time and searched for six hours before it returned to refuel. 

Maritime search authorities are often informed by medical experts as to survivability timeframes.

Factors include the time the man went overboard and the height of the fall. 

If the passenger survived, the water entry, wave height, currents and weather conditions would impact the search area and threat of hypothermia. 

Symptoms of hypothermia can develop within minutes, and at a water temperature of 32.5C, death may occur in under 15-45 minutes.

The April average for the northern Pacific is an estimated 26C. 

Cruise operator Royal Caribbean confirmed the guest had gone overboard during the trans-Pacific leg of the journey.

"The ship's crew immediately launched a search and rescue operation and is working closely with local authorities," a spokeswoman said.

Royal Caribbean was unable to confirm reports the man's partner had died on the ship after suffering a medical emergency. 

Cruise ship passenger Georgina Thompson told Nine Network she heard repeated calls of "Oscar" over the PA system. Oscar is an emergency code indicating someone has gone overboard.

"There were lights ... shining on the ocean and also there were a couple of boats out there in the water," she said.

"The light started searching along the outside of the ship.

"You just knew that something was wrong. My husband thought we might have hit something."

The ship is scheduled to arrive in Honolulu on Friday.