The accident occurred around 11:30 p.m. (1330 GMT) on Sunday near the town of Greta, about 180 kilometres (112 miles) northwest of Sydney, in an area famous for its vineyards and wedding spots.
At this stage it appeared to be a single vehicle collision, Chapman said, as police tried to identify all passengers.
Police said some people could be trapped beneath the bus, which was lying on its side.
The driver of the bus, a 58-year-old man, is under arrest and expected to be charged over the accident.
Heavy fog was present in the area when the accident occurred but Chapman said the cause of the accident had not been determined.
Persons:
Tracy Chapman, Chapman, Anthony Albanese, Renju Jose, Marguerita Choy, Stephen Coates
Organizations:
SYDNEY, NSW, Police, Thomson
Locations:
Hunter, Australia's New South Wales, NSW, Greta, Sydney