Dozens of senior Hong Kong officials and lawmakers were ordered into quarantine on Friday after a coronavirus cluster widened at a birthday party attended by many of the city's political elite.

The high-profile entries into Hong Kong's 21-day quarantine facilities have left city authorities red-faced as they prepare to install a new "patriots only" legislature, which is scheduled to hold its first meeting next week.

Senior cabinet members, including the city's police and immigration chiefs as well as 19 legislators, were among 170 people who attended a Monday night birthday bash for Witman Hung, a member of China's top lawmaking body.

A single preliminary coronavirus case was detected at the party sparking an initial run of quarantine orders on Thursday for a handful of people deemed to be close contacts. 

But health inspectors widened the quarantine order on Friday to all who attended after a second person at the event tested preliminary positive for the virus.

"All the people present at the party need to be quarantined," Chuang Shuk-kwan from Hong Kong's Centre for Health Protection told reporters.

The health department did not give a breakdown of the attendees headed to Hong Kong's quarantine camp.

But the government had previously confirmed 13 senior officials and 19 lawmakers made an appearance at the tapas bar where the birthday bash was held.

"We have more than 60 people at the quarantine centre now and the process is still ongoing," health official Edwin Tsui said.

One of the lawmakers, firebrand nationalist Junius Ho, met China's top official for Hong Kong policy across the border in Shenzhen two days after the party.

Patriots only' legislature 

While the party was legal under rules at the time, health officials had advised people three days earlier to avoid big gatherings.

Hong Kong leader Carrie Lam had previously said she was "very disappointed" that so many officials and cabinet members had joined a large gathering. 

Among those in attendance were Raymond Siu, the city's top cop, immigration chief Au Ka-wang, Home Affairs Secretary Caspar Tsui and the head of the city's anti-corruption watchdog.

"I offer my sincere apology to all people of Hong Kong. I have reflected on this incident and shall be more vigilant in future," Au said in a statement Friday. 

Lawmakers sent to the quarantine facility will now miss the legislature's first sitting next Wednesday. 

"I feel I acted carelessly and I rather regret it," lawmaker Duncan Chiu told local radio Friday morning. "This has set a bad example."

Multiple local outlets ran a photo showing birthday celebrant Hung singing into a microphone with his arm around the shoulder of a smiling woman, both unmasked. 

Hung issued an apology late Thursday and will be among those quarantined. 

President of the Legislative Council Andrew Leung said he hoped next week's meeting would continue as planned.

A new group of 90 city lawmakers were selected last month under new "patriots-only" rules imposed by Beijing, which vetted all candidates for their political loyalty. 

Only 20 were directly elected amid record low turnout, with the rest selected by reliably pro-Beijing committees. 

The government upped Covid restrictions this week, after a small outbreak of the Omicron variant was traced to a flight crew member from Cathay Pacific.

Earlier this week, Lam said the airline must take responsibility for aircrew breaching isolation rules even if "they may not know their employees' every act".

Asked by reporters on Thursday whether she should do the same, Lam said it would be "bad for governance" if she took responsibility for the behaviour of her ministers who were expected to lead by example.

It is not the first time officials have found themselves in hot water over health rules. 

Last year, three immigration officials, including Au, were fined after they broke social distancing rules to eat hotpot at a luxury clubhouse with a mainland property developer.