The government on Thursday put a public housing block in Yuen Long under lockdown after sewage samples taken from the building tested positive for Covid.
Officials said they suspect there are asymptomatic patients in Hung Lok House at Hung Fuk Estate, and the risk of infection in the area is likely to be higher.
Residents of the building are required to undergo Covid tests and stay at home.
Authorities said they expect to wrap up the operation by noon on Friday.