If confirmed, it would be the sixth case linked to the outbreak at the Chinese restaurant at Festival Walk.
Dr Albert Au of the Centre for Health Protection said the woman, 66, had lunch with eight relatives at Moon Palace on December 27.
A compulsory testing order is being issued for places she visited afterwards, including various eateries in Tai Wai and Sha Tin.
Dr Au also said health authorities are still trying to identify six of the 207 diners who were present at Moon Palace on that day, while the others were already sent to or being sent to quarantine.
“Our staff in the contact-tracing office… they will review CCTV in the mall in order to identify their whereabouts, and we will see whether we can chase them according to their Octopus record and other credit card records,” he said.
Health authorities also confirmed the fifth patient linked to the cluster: the wife of a construction worker who dined at Moon Palace.
The 38-year-old woman’s husband was confirmed with the coronavirus earlier.
Her nine colleagues at a Tsuen Wan office, as well as their 20 family members, are now being sent to quarantine.
Health authorities reported a total of 29 infections on Monday, with all but one being imported cases.
Among the latest cases is a Cathay Pacific aircrew member, 45, who lives in Tung Chung. The other patients flew in from countries such as the Philippines, Indonesia and Canada.
Officials also reported about 40 preliminary-positive cases.