There were also three cases that were deemed “indeterminate”.
Authorities said residents of Yiu Ping House in Yiu On Estate must get tested again for the virus, with the operation expected to end at 1pm on Friday.
The public housing block had been sealed off since Wednesday afternoon, after sewage samples there tested positive for the coronavirus.
A government spokesman called the discovery of preliminary positive cases “worrying” and said there’s a significant risk of infection at the building, so it is necessary to extend the lockdown to prevent the coronavirus from spreading further.