The territory has stuck to a zero-Covid strategy by largely isolating itself from the world and enforcing a draconian and costly quarantine regime.

On December 31, a streak of three months without community cases ended with the first local transmission of the new Omicron variant.

Since then, authorities have scrambled to track down and test hundreds of people who had been in contact with a handful of Omicron patients.

"The most stringent anti-epidemic measures will be implemented to prevent the mutant strains from spreading," the government said in a statement.

The latest hunt was sparked by a patient who danced with some 20 friends in a central park on New Year's Eve. Two of the fellow dancers, one of whom was a domestic helper, came up positive in preliminary tests.

The helper's employer and eight other of her close contacts then went on a cruise journey on January 2.

As part of its coronavirus restrictions, Hong Kong has restricted cruises to short trips in nearby waters, with ships asked to operate at reduced capacity and to only allow vaccinated passengers who test negative for the virus.

The "Spectrum of the Seas" ship, which returned a day early, had about 2,500 passengers and 1,200 staff on board. The nine close contact passengers were isolated from the rest of the people on board and preliminary tests taken during the journey returned negative results, authorities said.

"Spectrum of the Seas is taking appropriate measures under guidelines by the Department of Health," Royal Caribbean told Reuters in a statement.