A family living in Tung Moon House at Tai Hang Tung Estate have been infected after their relative, who had flown in from Pakistan, came down with the virus after completing her hotel quarantine.
Health authorities said a woman, 42, and her daughter, 3, who live in the flat directly above have now tested preliminary positive.
Yuen said the virus might have been transmitted by exhaust fans in flat 514 to flat 614.
But he also said the virus could have spread through sewage pipes.
About 30 residents living in flat number 14 from the first to the 11th floor were being urged to evacuate.
The woman and her child had tested negative during a previous overnight lockdown of the building, but the child had developed a fever since.
The Centre for Health Protection said more testing locations will be set up in Sham Shui Po for residents there to get tested.