It said about 270 residents of Jovial Court in Peninsula Village had been tested as of midnight.
The lockdown was triggered on Sunday, after a 23-year-old man returning to Hong Kong tested preliminary positive for a mutant strain of Covid-19.
His sample had tested negative on November 13, a day before he left for the US, but he tested positive when he returned to the SAR.
He had had two doses of the BioNTech vaccine in April and May, and showed symptoms.
Last updated: 2021-11-22 HKT 07:03