The bus, going from Bengkulu to Palembang on Monday, hit a guard rail at a bend shortly before midnight causing it to plunge 150 meters into the Lematang River, the police said in an incident report.
Another 13 people were injured in the accident, head of Pagaralam Police Department Dolly Gumara told Efe on Tuesday, adding that some victims were still being identified.
The injured were taken to Besemah Pagar Alam Hospital.
"What we're doing now is we're still evacuating and finding victims who might be on the slope and the river," Gumara said.
He said the cause of the accident was not yet known.
"The cause of the accident still needs investigation and the South Sumatra Head of Traffic is on his way here with his team. One cause is because of the difficult terrain," Gumara said, adding that the 'curved road' was another possibility.
Social media videos appeared to show a steep downhill bend on a road where emergency vehicles had gathered.
It is unclear how many passengers were in the bus as it was suspected that people had been picked up along the route.
Indonesia has seen a number of fatal traffic accidents in the past fortnight, according to a media report.
( With inputs from IANS )