The recent cluster of nine patients surfaced on Sunday when officials confirmed the infection of a 34-year-old woman, who works at a Kerry Logistics warehouse in Kwai Chung. It later expanded to include her 56-year-old husband, two of her colleagues, and the paramedic who took her to hospital.

Of the six infections reported on Tuesday, four were linked to the cluster, all from Luk Chuen House in Lek Yuen Estate in Sha Tin, and neighbours of the woman. The other two infections involved a boy returning from Pakistan and his close contact.

The health department said that as of 4pm on Wednesday, it had collected 1,056 test bottles from residents of the block. Some 1,031 deep throat saliva samples were tested and all were negative for the virus except four previously announced cases.

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But 25 samples needed to be resubmitted because of leakage, and residents in 66 flats had yet to return their bottles. The Centre for Health Protection urged them to return their bottles as soon as possible. “The CHP will arrange compulsory testing for those who do not return their samples,” it said.

The centre said on Wednesday night that 31 of the 66 households would submit their samples, while 13 families had moved out in mid-May.

But health officers had not yet managed to contact the remaining 22 households. Letters had been left at those flats, urging them to contact the centre and return test samples as soon as possible.

Epidemiologists had warned of potential “super spreading”, with lifts and postboxes being possible routes of transmission, as some residents fled the building.

University of Hong Kong microbiologist Dr Ho Pak-leung had called for mandatory testing.

“The authorities should enter homes of those who have not returned samples. If they are too passive and just sit back, it does not work,” he said, adding asymptomatic carriers might pass on the virus.

Infectious disease expert Yuen Kwok-yung, who sits on a Covid-19 advisory panel reporting directly to the city’s leader, agreed the health authorities should conduct mandatory testing if they had the legal power to do so.

But Secretary for Food and Health Sophia Chan Siu-chee sought to diffuse concerns on a radio programme earlier on Wednesday.

“All samples collected from the environment tested negative … residents do not need to be overly worried,” she said.

Both experts also renewed their call for the city to scale up testing capacities.

Hospital Authority and health department laboratories carry out about 2,000 Covid-19 tests daily and Chief Executive Carrie Lam Cheng Yuet-ngor had promised to increase that to 7,000. However, Professor Gabriel Leung, dean of HKU’s faculty of medicine, on Tuesday called for 10,000 tests.

Yuen said 7,500 tests per day seemed “most reasonable”, or one per 1,000 people in the city.

“The issue is that the government has not acted sufficiently and has not yet facilitated all these to happen after four months.”

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Ho said among the 73 government clinics which were open for residents to hand in samples for testing, the average daily intake last month was only 170.

He urged the government to extend the opening hours for stations collecting samples, and even set up street booths for public convenience.

Alex Li Wai-chun, chairman of the Hong Kong Association of Medical Laboratories, said the current daily capacity of the seven private labs that did Covid-19 tests was between 500 and 1,000. He believed the figure could be doubled.

“But the private sector faces three difficulties – space constraints for new machines, the availability of test kits and manpower.”

Dr William Ho Shiu-wei, chairman of the Hong Kong Private Hospitals Association, which represents 12 institutions, said there was not a lot of demand for private testing at the moment as patients could get it done for free from the government.

But he anticipated a surge in demand for a “negative Covid-19 certificate” from cross-boundary travellers when borders with mainland China were reopened.

Meanwhile, the Education Bureau revealed that up to 2,500 senior secondary cross-border pupils – Hongkongers living on the mainland – could be back to school as early as mid-June.

A source said the bureau had notified secondary schools that they could tender for bus companies to provide services directly from border checkpoints to campuses, with a suggested one-month contract period to begin from June 11.

Face-to-face lessons for local students began in phases from last Wednesday, but as many as 27,000 cross-border pupils had to resort to online learning at home as both the Hong Kong and Shenzhen authorities were still discussing quarantine and transport arrangements.

Additional reporting by Chan Ho-him