The city marked a third straight day of 100 or more new infections, with existing clusters, including ones involving a construction site in Tseung Kwan O and a department store in Sha Tin, continuing to grow.

Professor Yuen Kwok-yung of the University of Hong Kong inspected Block 6 of Richland Gardens on Thursday with government officials.

“We found that the pipes in the kitchen, the toilet and the bathroom are all connected to the main sewage pipe, but we were not certain during the inspection if there were any U-shaped water traps at some flats. We have reached the conclusion that there is a safety risk for residents living in flats facing the same direction.”

Yuen, who also advises the government on its pandemic response, added: “All residents in units D of Block 6 should be sent to a quarantine camp for isolation. We are worried that if they stay here longer they will be in danger. This housing block is now an epidemic centre.”

Health officials said fewer than 100 residents were being evacuated on Thursday.

One more person in a D flat of Block 6 was found to be preliminary-positive on Thursday after 900 residents in the 34-storey building were ordered to undergo testing on Wednesday night.

The new case would bring to eight the total number of infections in the block, including three people from the same household on the second floor. The first infection from that floor was confirmed on November 23. Infections on the third floor then emerged, followed by another on the 34th floor, with the latest confirmed case, on level six, recorded on Wednesday.

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The estate also recorded one case in a D flat of Block 18 and one in an E flat of Block 4 in late November. Residents in those flats on all floors of the two blocks would also be ordered to get tested.

Yuen said other possibilities could not be ruled out, as the virus could also spread through other means, or through an effect called “upstream influence” where air moves upwards against a wall.

Kenneth Leung Kai-ming, from the Environmental Protection Department, said officers would carry out further inspections with the Buildings Department to see if any parts of the drainage system had been altered. Initial inspections did not find any damage to pipes.

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Fourth wave of coronavirus cases in Hong Kong prompts tougher Covid-19 measures

Fourth wave of coronavirus cases in Hong Kong prompts tougher Covid-19 measures

Yuen advised the public to get professionals to check their drainage system if they noticed a faecal smell inside their flat.

“Before a fix is done, wear a mask when going to the toilet,” he said, noting that adding detergent to the toilet bowl before flushing could help prevent the virus spreading.

Building surveyor Vincent Ho Kui-yip said the drainage system in Richland Gardens used a common design from the 1980s, in which all discharge pipes from the kitchen and bathroom were connected to one main “soil and waste pipe”. Newer buildings had a two-pipe system separating sewage from waste water. “But the design itself is not a problem. The problem could be in alteration works for the bathtub,” Ho said.

Some residents liked to convert the original bathtub into a shallower one or a shower, he said, and in the process, workers could have failed to install a “bottom trap”, which, similar to toilet bowl U-traps, retained water and prevented virus-contaminated air or vapour from spreading.

A similar problem was identified in Amoy Gardens, he said.

It was not until the 2000s that regulations required a bottom trap in bathrooms.

Asked if Block 6 should be placed in lockdown, Yuen said the city’s systems might not be able to cope, as lots of support would be needed for those in the building. The limited amount of quarantine places also meant it was not possible to send everyone from the block to government facilities, he said.

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A resident surnamed Lau living in a D flat in Block 6 said he regularly poured bleach down the U-bend in his toilet and had kept his windows closed, so he was not particularly worried.

“If the government wants to put us under lockdown, they need to provide us with help such as food, they can’t just lock us in our homes like sitting ducks,” he said.

Another resident, surnamed Chu, expressed concern over the rising number of new cases. “Even if we need to go into lockdown, it is fine if it can control the number of cases here,” she said.

Dr Joseph Tsang Kay-yan, an infectious disease specialist, said the government should lock down buildings where mandatory testing orders had been issued.

“The government needs to let the public know when the mandatory testing or lockdown orders will be implemented if there are more than a certain number of infections in a building,” Tsang said. “You need to let the public know earlier so they can be ready.”

Undersecretary for Food and Health Dr Chui Tak-yi said he was confident mandatory testing would quickly identify the degree and pattern of the virus spread at Richland Gardens, and that was the main strategy the government would adopt.

“There is no single numerical quantifier for any government to introduce a lockdown measure for a particular place or building, it is only one of the instruments that can be used,” he said.

Additional reporting by Victor Ting and Joyce Ng