KMB bus 872 rolled over during the incident on Tai Po Road on Saturday evening, though the cause of the crash remains unclear.
Local media reports quoted passengers as saying the crowded vehicle tipped over after it had been traveling too fast.
Fourteen people were injured last April when a double-decker tram tipped over, with a 23-year-old driver later arrested for unsafe driving causing harm to others.
A representative for the Kowloon Motor Bus Co, the bus operator, said it would launch its own investigation into the crash and family members of the victims would get financial assistance.
The accident was reported at around 6pm local time after the bus flipped on its side while going through Tai Po Kau, Tai Po Road, passing by a nature reserve.
The accident was the city's deadliest bus incident in almost 15 years.
Most of the dead and injured were on the upper deck of the bus, Chan Hing-yu of the fire department told reporters.
Firemen work at the scene of a bus crash in Hong Kong Saturday Feb. 10 2018
Earlier reports said that 19 people were killed.
One passenger was sent to Prince of Wales Hospital in Sha Tin in a "half-conscious" state, according to the force.
The bus had been running on high speed before the accident, according to some of the passengers.
Emergency workers ferry bodies from the scene of the horrific Hong Kong bus crash.
The driver had been with the company since 2014 and was familiar with the route on which the accident happened. So far there is no sign of fatigue driving when the accident occurred, said the general manager.
The Hospital Authority said 19 people were in a serious condition.
In September previous year, three people were killed when a double-decker bus mounted a pavement in Sham Shui Po.
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