The government on Wednesday extended the lockdown of a public housing block in Sheung Shui for another day, after a mass testing operation overnight found more than 30 coronavirus infections.
One case was described as "indeterminate".
Ching Long House on Ching Ho Estate has been sealed off since Tuesday evening after authorities detected the virus in sewage samples there.
A government spokesman said the discovery of the positive cases was worrying.
The spokesman said the infection risk at the building was very high and the lockdown had to be extended until noon on Thursday to prevent the virus from spreading.
Residents will be tested again for Covid-19.
The government said anyone who was in the building for more than two hours in the past two weeks must also get tested.
People who do not comply with the testing order risk a fine of HK$10,000, it added.