According to the police the incident occurred along the Butwal-Palpa road section of the Siddhartha Highway at Siddhababa in Palpa district.
The death toll may rise as the bus is completely submerged under the water.
Some passengers escaped the incident when they saw the land above the road falling and jumped off the bus.
The bus carrying at least two dozen passengers onboard was heading towards Butwal from Palpa district when the accident occurred.
The police said that some 15 passengers were rescued from the incident site.