Four residential blocks in Kwun Tong, Aberdeen, Sham Shui Po and Tsuen Wan have been sealed off for overnight testing after their sewage samples tested positive for the coronavirus.
Officials said there could be infected people without symptoms in Tsui Yue House of Tsui Ping (North) Estate, Lei Chak House of Ap Lei Chau Estate, Cheong Chung House of Nam Cheong Estate and Kwai Shue House, Lei Muk Shue (Two) Estate.
The government said anyone else who had been in one of the buildings for more than two hours between March 11 and March 24 will also need to get a Covid test, unless they have tested positive for the virus in the past three months.
Meanwhile, four other lockdowns that started on Wednesday afternoon have identified nearly 200 Covid cases.
More than 1,200 people were tested in Yau Ning House of Tin Yau Court in Tin Shui Wai and 38 results came back positive.
The lockdown of Tsui Mui House of Tsui Ping (North) Estate in Kwun Tong turned up 65 infections, after the testing of about 900 people there.
In Wong Tai Sin, Kwai Tung House of Tung Tau Estate was unsealed after more than a thousand people were tested and 48 of them found to be Covid-positive.
The authorities found another 43 cases after testing more than 1,100 people from Chui King House of Choi Hung Estate in Wong Tai Sin.