Authorities on Saturday sealed off two public housing blocks in Sha Tin, after sewage samples tested positive for Covid.
The buildings are Pok Tat House, on Pok Hong Estate; and King Wo House, on Wo Che Estate.
The government said it aims to finish the exercises early on Sunday afternoon.
Meanwhile, 150 preliminary positive cases have been found following a lockdown last night at a public housing block in Shek Kip Mei.
The operation at Block 21 of Shek Kip Mei Estate will now be extended until Sunday afternoon. Residents must now get tested a second time.