Earlier this week, the front page of the influential local newspaper Apple Daily criticized expatriates gathering in a bar without masks. The story tapped into a widespread perception among many Hong Kongers, whether true or not, that some of the city's foreign residents have not shared their alarm at the outbreak.

Some residents were frustrated at the attitude of a minority of expatriates, including some bosses and colleagues, who belittled or mocked the emergency measures being put in place -- such as wearing masks everywhere or working from home -- ignoring the painful lessons that their locally-born colleagues had learned from SARS.

There was also a feeling that many foreign passport holders had an easy escape route to other countries that Hong Kongers did not.

With Hong Kong now among the safest places in the world to be, some who left are starting to return, while others are being forced to -- many countries which attract large numbers of Hong Kong students are closing universities and going into lockdown -- raising concerns they will bring the coronavirus back with them.

On Tuesday, the city's government issued a red notice for all foreign countries, requiring anyone arriving from overseas to undergo a 14-day home quarantine. Of the 10 new cases reported that day, almost all had recently traveled in Europe or the United States.

"In view of proliferation of the disease and continuous increase in the number of cases reported around the world, members of the public are strongly urged to avoid all non-essential travel outside Hong Kong," the city's Center for Health Protection said in a statement.

"The CHP strongly urges the public to maintain at all times strict personal and environmental hygiene, which is key to personal protection against infection and prevention of the spread of the disease in the community. On a personal level, members of the public should wear a surgical mask when having respiratory symptoms, taking public transport or staying in crowded places. They should also perform hand hygiene frequently, especially before touching the mouth, nose or eyes."

Speaking this week, Hong Kong leader Carrie Lam said that "if all these imported cases lead to a community outbreak, the consequence will be unimaginable and increase the burden on the public health system."

With many people having returned to the city before this measure was put in place, there are concerns not all will voluntarily abide by the government's warnings.

As unconfirmed claims spread on social media alleging returnees were ignoring quarantine measures, some businesses are taking matters into their own hands. Hemingway's, a bar in the city's Discovery Bay area, which has a large foreign population, on Monday issued a "warning to anyone returning from Europe."

With the fear that imported cases could lead to a second wave of infections, graffiti urging people to wear masks has become a common sight in some parts of the city. "Hey gweilo, too poor to buy a mask?" reads one such piece. Gweilo is a Cantonese word commonly used to refer to foreign residents.

Part of the problem is that, while Hong Kong has long advised people to wear face masks in public, other governments have given conflicting advice, and those returning to or going about the city unmaksed may be following the guidance of their home countries.

She pointed to Hong Kong as an example of somewhere masks were common, and infection rates low. Officials must now reinforce that message in Hong Kong, or risk undermining the city's own hard won safety.