The Norwegian Jewel had recently visited New Zealand.
Owners of the Norwegian Jewel cruise ship say some passengers contracted "stomach-related illnesses" - but there is no coronavirus scare onboard.
The cruise ship arrived in Sydney today and Australian media reported that it was placed in lockdown while authorities tested a sick passenger for coronavirus.
The ship was in New Zealand for a 10-day tour before arriving in Sydney today.
It is unclear where the man, who Australian media reports is Singaporean, boarded the vessel.
He fell ill with respiratory illness, The Australian reported.
The man was being tested as a precaution and had not yet been confirmed to have
coronavirus.
The ship's owner Norwegian Cruise Line dismissed Australian reports that a respiratory illness had occurred onboard.
"The safety, security and well-being of our guests and crew is our number one priority," the company said.
"As such, we operate at the very highest public health standards. During Norwegian Jewel's sailing from Auckland to Sydney, a few guests on board experienced a stomach-related illness.
"To mitigate any impact of this rare occurrence, we implemented stringent sanitation procedures. As such, upon the ship's return to Sydney [today] a a thorough inspection and rigorous cleaning and disinfection of the ship and terminal were conducted."
Reports suggesting a coronavirus scare were false and inflammatory, the company said.
"The vessel remains in operation, and all guests onboard are in good health."
The cruise ship was last in New Zealand in February 10 at Port Chalmers near Dunedin.
The ship had also visited Auckland, Tauranga, Gisborne, Wellington and Akaroa.
The cruise ship hit the headlines during an earlier New Zealand visit in December when a man jumped over board near Mahia Peninsula and was never found.
CCTV footage showed the 62-year-old Australian jumping overboard from the Norwegian Jewel.
The 294-metre ship has a guest capacity of 2376 people and 1060 crew.
A 29-year-old woman was charged with wilful damage after the incident.