What are the requirements of the Prevention and Control of Disease (Wearing of Mask) Regulation (Cap. 599I)? Cap. 599I empowers the Secretary for Food and Health, for preventing, protecting against, delaying or otherwise controlling the incidence or transmission of the specified disease, to, by notice published in the Gazette, impose compulsory mask wearing requirement during the specified period.

https://www.coronavirus.gov.hk/eng/public-transport-faq.html