A mass Covid-19 testing exercise at Mei Wui House in Shek Kip Mei Estate found 70 preliminary positive cases, while eight results were uncertain.
Residents must get tested again for the virus, with the operation expected to end by Saturday afternoon.
The public housing block has been sealed off since Thursday evening, after sewage samples there tested positive for the coronavirus. Authorities had also discovered a preliminary positive case.
A government spokesman 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.