The lockdown, at the Cheung Hing Building on Pitt Street, was ordered after a cargo aircrew member tested preliminary positive for Covid – a case which officials fear may involve the Omicron variant.
In a statement, it said the lockdown began at 7pm on Thursday and as of midnight, about 160 residents had undergone testing, with no confirmed cases found.
It added people in the restricted area, who had undergone testing and could provide a negative test result, could leave through a “designated exit” from 7 am.
The case, involving the Cathay crew member, prompted calls for the government to consider scrapping a quarantine exemption for cargo crew.
Officials said the crew member, 36, left for the United States on Saturday and flew back to Hong Kong two days later.
Tests taken before he left Hong Kong and after his return came back negative.
He then stayed at home for most of the time for medical surveillance, except from going out to buy food briefly on Wednesday and getting tested at a mobile specimen collection station.
That test came back preliminary positive with the N501Y mutant strain detected. The man also started developing symptoms with a Ct value about 25 to 29.