A gastro outbreak on the Sun Princess cruise ship has affected 176 people, with cases having already peaked, Queensland Health says.
The 15-deck, 2000-passenger vessel is undergoing a "deep cleansing" after docking in Brisbane on Thursday morning.
The norovirus illness struck down 173 passengers and three crew members during the 14-day voyage to New Zealand.
The first cases were recorded on February 3 and were now in decline, according to Queensland Health.
The operator, Princess Cruises, has said "most guests" were unaffected, with 12 to 15 of the 2000 in self isolation on any given day during the trip.
It is the second gastro outbreak aboard the Sun Princess in two weeks, with about 90 people becoming sick on a trip to Papua New Guinea in early February.
Public Health Physician Dr James Smith said it was possible the virus was reintroduced by a passenger not on a previous voyage, although it was difficult to know what the source was.
"(It) is an extremely hardy virus, it can survive on hard surfaces ... for quite a long period of time, even over a month," he said.
"It also spreads very easily.
"It's very, very hard to get rid of."
Queensland Health cannot shut the cruise ship down but can provide support and advice to its operators.
It has sent in two environmental officers to help with sanitary precautions on the ship in a bid to eradicate the virus.
One passenger, Kathryn Perrott, told the ABC she decided to fly home from Auckland rather than travel back with the ship because she was concerned about the growing outbreak.
"We had cabins all around us being cleaned by people in hazmat suits, with people obviously sick," she said.
Ms Perrott said the captain announced a spike in the virus over the ship's PA system.
"We'd walk down the long corridor, go into our room and hear people throwing up as you're going down the corridor. It's not very pleasant," she said.
Norovirus symptoms typically present between 24-48 hours and generally includes nausea, diarrhoea, vomiting and some cramping.
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