All residents living in a Kowloon Bay housing block facing the same direction will be moved to quarantine centres after a government advisor and University of Hong Kong microbiologist, Yuen Kwok-yung, said the Covid-19 virus might have spread through the building's sewage pipes.
Meanwhile, residents at two other nearby buildings in the same estate have been ordered to get tested for the virus.
There have been seven confirmed cases at Block 6 of Richland Gardens, and Yuen said it may be unsafe for residents of unit D to stay.
"We know that all the cases are in unit D of Block 6, and as a result there is a good reason for us to suspect a structural problem, especially those related to the sewage system," Yuen told reporters after inspecting the building on Thursday.
"All the floors' drains, from either the bathroom, or the toilet, or the kitchen all drain into the sewage pipe, and [in] some of them we may not be able to see there's a U-trap," he said, referring to the pipe design that wards off germs and viruses.
Yuen said the drainage pipes were designed similarly as those in Amoy Gardens, which was at the centre of the Sars outbreak in 2003.
But he said the situation in Richland Gardens is less dangerous because the yard here isn't enclosed like in Amoy Gardens.
The renowned microbiologist also said the authorities will decide if the entire Block 6 has to be sealed off or evacuated completely after residents get tested for the virus as ordered by the government.
Health authorities said later on Thursday that everyone who lives in the 'D' units of Block 4 and unit E of Block 18 must get tested because those two buildings have the same design as Block 6.
The controller of the Centre for Health Protection, Wong Ka-hing, said a rapid antigen test that identifies patients with a high viral load in 20 minutes will be conducted for the residents.
But he said people still have to wait until their nucleic tests return negative before they can be sure they are clear of the virus.
Workers from government labs and testing contractors were seen in the estate on Thursday morning to help set up a testing station at a badminton court there.
A resident who lives in Block 6 of Richland Gardens said he had leaky drainage pipes in the past, but they have been fixed so he's not too concerned about the virus spreading.
But another resident, surnamed Chu, said she's worried about the Covid cases in her building, adding that she would not mind a lockdown if more residents become infected.
A Ms Ho who also lives in Block 6 says the arrangement is confusing. She said she knew all residents in the block had to be tested, but authorities at the same time told people not to leave home. So, she was not sure when was the right time to get tested. She only found out testing booths have already been set up when she finally went downstairs to check.
A resident living in another block voiced concerns about communal facilities in the area, saying a shopping mall nearby might be contaminated.
Earlier in the year, the authorities said a cluster at Hong Mei House in Tsing Yi during an earlier wave of coronavirus cases could be due to vent pipe faults.
Last updated: 2020-12-10 HKT 18:15