The man was said to have traveled to the United States earlier and had stayed in the US for over 14 days. His Covid test at the Hong Kong International Airport came back positive when he returned to the city recently.
The 52-year-old is now receiving treatment at the North Lantau Hospital Hong Kong Infection Control Centre.
Health authorities are looking into whether the man had contracted the virus in the US.
Sources also told that before the man departed Hong Kong, he had stayed in the city with another incoming traveler.
Natural phenomenon, also known as algal blooms, sighted in several coastal areas including Tuen Mun, Tsuen Wan and Tung Chung recently.