Health authorities on Wednesday found 11 Covid cases from two overnight lockdowns in Sha Tin and Fanling.
Eight of the infections were picked up at Shou Chuen House of Shui Chuen O Estate.
The remaining three infections came from Cheung Fung House of Cheung Wah Estate, after screening more than 790 people there.
The housing block in Fanling also found one undetermined case.
Separately, officials sealed off two buildings on Wednesday afternoon in Fanling and Wong Tai Sin for compulsory testing.
Residents at Cheung Yue House of Cheung Wah Estate and Block 10 of Tsui Chuk Garden are required to get tested before 9pm.
The overnight lockdowns came after the government said it has reviewed “a basket of factors”, including the viral load in sewage samples and positive cases linked to the buildings.
The operations are expected to be completed at 1pm on Thursday.