The girl, 17, lives in Shing Yu House at Tin Shing Court.
She developed symptoms including fever, runny nose, headache and loss of smell on Wednesday.
A saliva sample she submitted was found to carry the N501Y mutant strain of coronavirus.
The girl is a student of Queen Elizabeth School Old Students’ Association Tong Kwok Wah Secondary School in Tin Shui Wai and has attended tutorial classes at Tak Wing Industrial Building in Tuen Mun.
She also visited an education expo at the Hong Kong Convention and Exhibition Centre in Wan Chai last Saturday.
People who’ve been to these places will also be ordered to get tested for Covid-19.
The Centre for Health Protection said the girl and her family members have not been vaccinated against coronavirus.
Her family members and other close contacts were put under quarantine for 21 days.
Residents of Shing Yu House are required to get tested for Covid before they are allowed to leave the building on Saturday.
They will also be tested again four more times in the next two weeks as a precaution.