Residents and recent visitors to Block One of Bel Air Heights and Ching Chuen House of Shui Chuen O Estate must get tested.
Candidates of the Diploma of Secondary Education (DSE) public exam and exam personnel will get priority for the tests by showing their exam admission forms and ID cards.
Officials hope to complete the exercises around mid-day on Friday.
Meanwhile, a lockdown of Shun Hang House of Shun Lee Estate in Kwun Tong the previous night picked up nine infections and three uncertain results, among nearly 1,400 people tested.
Officials are also handing out some 240,000 at-home rapid test kits in Tseung Kwan O, Tuen Mun and Fortress Hill, after sewage samples in the areas tested positive for the coronavirus.