Hong Kong has ended its two-month streak of not having a local Covid-19 case, as it steps up surveillance to also prevent imported cases from the Chinese mainland and Macau.

In a statement issued yesterday, the Centre for Health Protection said the city has recorded one new local case - a 43-year-old welder - and another five imported cases.

It said the man had tested negative under the regular testing requirement in place since May.

"After admission to hospital on Aug 3, the patient tested positive for antibodies against Covid-19," it said.

"After examining the epidemiological, clinical and laboratory findings, this case is compatible with a 're-positive' case previously undiagnosed," it said.

"Re-positive" refers to cases in which a patient who had previously recovered from Covid-19 tests positive again.

Hong Kong has recorded more than 12,000 cases and 212 deaths.

The health authorities said on Wednesday that the welder, who is unvaccinated, lives in the Sham Shui Po district, works in the Wan Chai district and has no recent travel history.

They also said that a 13-year-old girl and her family would be quarantined after they found the child to be a close contact of a patient in Macau who was infected with the Delta variant of the coronavirus.

The girl and the Macau patient had gone on a dancing trip in the Chinese city of Xi'an last month. She returned to Hong Kong on Monday and tested negative on Wednesday.

Overnight, the authorities locked down a Sham Shui Po block of flats and a hotel in the Tsim Sha Tsui area to get residents there tested. These places were linked to the welder and two imported cases on Wednesday.

The government also issued compulsory testing notices for 22 locations, including two construction sites in Wan Chai, a Fairwood restaurant in the Shek Kip Mei area, a Wellcome supermarket in the Jordan area and the Immigration Tower in Wan Chai.

After an almost two-month virus-free period, officials on Monday had announced more relaxed quarantine rules for vaccinated people entering the city from medium-risk areas.

But on Wednesday, Hong Kong tightened border controls to insulate itself from the worsening outbreak on the mainland.

The city suspended quarantine exemptions for residents returning from the mainland, with the exception of Guangdong province. The seven-day quarantine for those fully vaccinated no longer applies. Instead, they will serve 14 days in home quarantine.

Under the quarantine exemption scheme, up to 6,000 residents could return to Hong Kong daily from the mainland without having to go through quarantine.

Also on Wednesday, the government's decision to list Macau as an at-risk place took effect.

Those who have been in Macau on the day they arrive in Hong Kong or during the 14 days before that will be put on 14-day home quarantine and subject to Covid-19 testing.