The 43-year-old woman, a horse owner who lives at Tower 2, Harbour Green, in Tai Kok Tsui, had drinks with an infected guest - 37-year-old Chinese Manufacturers' Association of Hong Kong's youth committee member Celia Wong Sze-nga - before she tagged along with her to the party on January 3.

The woman went to work at the Cable TV Tower in Tsuen Wan after the party and had dinner at the Hong Kong Jockey Club in Happy Valley on January 5.

She was not on the guest list but went there after 9.30pm, along with Wong, the first confirmed case from the party. But she was not sent to quarantine even because Wong reportedly did not tell health authorities about their get-together.

Centre for Health Protection's head of the communicable disease unit, Chuang Shuk-kwan, said authorities did not know Wong had a meal with others apart from joining the party.

Authorities will take action if they find out Wong was lying about her whereabouts, Chuang said.

It is understood that Wong's family owns Wing Yuen Tea Co and that she has a "close relationship" with government officials and politicians.

Chuang said the party was attended by 192 guests, according to a list provided by NPC deputy Hung, and that 181 of them have been contacted so far.

She appealed to all attendees to get tested and contact authorities as soon as possible.

Those who fail to comply with a compulsory testing notice can be fined HK$25,000 and jailed for up to six months, she warned.

The Jockey Club said it tracked the visiting records of the 43-year-old woman and identified two members who had contact with her and made bookings at Sha Tin Racecourse yesterday. It added: "The club has immediately contacted them and required one of them not to attend the race meeting and the other to leave the racecourse immediately."

One of those at the party, Secretary for Home Affairs Caspar Tsui Ying-wai, said he has been "deeply reflecting on his mistakes" on the fourth day of his 21-day quarantine at Penny's Bay.

Director of Immigration Au Ka-wang and the political assistant to the secretary for development, Allen Fung Ying-lun, are also at Penny's Bay as they left the party after 9.30pm that night. Au could not provide proof of his whereabouts.

Tsui said on Facebook that he is not on holidaying in quarantine: "While in isolation, I've made deep self-reflection and reconstructed the events to evaluate what I could have done better.

"As the secretary for home affairs, I should constantly stay in touch with people from different sectors. I also play a part in the SAR's anti-epidemic work [and did not think through the ramifications of attending the party].

"Looking back, being an official is not like being an ordinary citizen and I should have higher standards for myself. At this critical moment in fighting the pandemic, I should have heightened alertness and rejected the invitation in a polite manner."

Tsui said he could not change what had happened and apologized for letting people down.

Tsui also said he has been taking Covid tests every day, with all results coming back negative.

His subordinate, undersecretary for home affairs Jack Chan Jick-chi, also apologized yesterday, the last official to issue an apology statement among the 13 who attended the party.

Chan said he left around 7pm and did not have to be quarantined at Penny's Bay, adding he has also tested negative.

A hundred lawmakers and officials who left the party before at 9.30pm were allowed to leave the quarantine camp on Saturday after the case of RTHK host Joyce Yeung - another attendee - turned out to be false positive.

Those who left included Secretary for Financial Services and the Treasury Christopher Hui Ching-yu, Police Commissioner Raymond Siu Chak-yee, ICAC Commissioner Simon Peh Yun-lu and lawmakers Lai Tung-kwok, Junius Ho Kwan-yiu and Kenneth Fok Kai-kong.

But 92 people who stayed after 9.30pm, including Tsui and Au, have to complete 21 days at the camp.

Chief Executive Carrie Lam Cheng Yuet-ngor required all 11 government officials who left the quarantine center earlier to isolate themselves at home using their leaves. They will still have to undergo tests on January 10, 15 and 22, and can only return to work after the results return negative.

Meanwhile, a picture circulated online showing 42 Lands Department staff taking photos together without masks, triggering suspicions they violated social distancing measures. The department said it was taken during a ceremony on January 5 and that staff took their masks off only during the picture-taking.