More than 180 passengers and crew on a Royal Caribbean cruise liner that has docked in Sydney Harbour have been struck down by a gastro outbreak.

A stomach bug has struck down more than 180 cruise ship passengers and crew on a Royal Caribbean liner that has docked at Sydney's Circular Quay.

The Explorer of the Seas arrived at around 6am on Wednesday with Royal Caribbean confirming 182 cases of a gastrointestinal illness among guests and crew of the 14-night trip.

"Those affected by the short-lived illness have responded well to over-the-counter medication administered onboard the ship," the statement issued on Wednesday said.

Paramedics were on stand-by as the 3566 passengers and 1139 crew members disembarked the Royal Caribbean Explorer, but no patients needed to be transported, am Ambulance NSW spokesman told AAP.

The ship and terminal will undergo enhanced cleaning and sanitisation to prevent any illness affecting future cruises as the ship prepares to depart for its next voyage on Wednesday evening.

All new guests embarking will receive a letter at boarding that asks them to advise if they have experienced any gastrointestinal symptoms within the last three days, Royal Caribbean said.

With more than three per cent of the ship's passengers struck down with the stomach bug, it is a legal obligation that health authorities be alerted.

The outbreak started on December 5 and peaked on December 11 and 12, but has since been decreasing, NSW health authorities told AAP.