Ten of the injured were in critical condition, while another 20 were seriously wounded, officials said.
Firefighters on the scene cut open the bus roof to free passengers still trapped inside.
Chief Executive Lam Cheng Yuet-ngor speaks to reporters in the Prince of Wales Hospital after visiting the injured of the earlier bus overturns on February 10, 2018.
The dead included 17 males and two females, police said.
Hong Kong implemented guidelines in 2010 that limit drivers' shifts to 14 hours total and 11 hours behind the wheel, but officials said after a crash past year that more wide-reaching policies would be necessary to improve safety and working conditions, the Times reports.
Police are still investigating the cause of the accident, she said.
The injured have been rushed to hospitals, while the site of the accident has been cordoned off. And then it was like the tyres slipped and the bus turned. "People fell on one another and got tossed from side to side".
While bodies of dead passengers were laid out on the ground, covered in white sheets. The fire brigade responded at 6.15pm local time. One man told the outlet as he sat on the roadside with his head wrapped in a bandage that the driver had seemed as if he were flying an airplane "around the bends".
An executive surnamed So, with The Kowloon Motor Bus Co that operated the bus, said $HK80,000 ($13,094) in financial assistance would be paid to the families of each victim.