The passengers were returning from a wedding at a winery on Sunday night when their coach overturned on Wine County Drive near Greta in Hunter Valley, New South Wales.
Police say they are investigating the circumstances surrounding the crash.
The driver of the bus, a 58-year-old man, has been arrested.
NSW Police Acting Assistant Commissioner Tracy Chapman said the guests were travelling to Singleton "presumably for their accommodation". The crash occurred around 23:30 local time.
The number of fatalities could increase, with 25 passengers taken to hospital - two of them airlifted from the crash, Ms Chapman said. She added that the bus is still on its side and people could be trapped underneath.
Ms Chapman said there was "sufficient information" for police to charge the coach driver.
"He's under arrest. He's been the driver of a motor vehicle collision where there have been fatal injuries and there will be charges pending," she said.
Australian Prime Minister Anthony Albanese said it is "so cruel, so sad and so unfair" for a "joyous day in a beautiful place like that to end with such terrible loss of life".
"People hire a bus for weddings in order to keep their guests safe, and that just adds to the unimaginable nature of this tragedy," he said at a press conference in Canberra.
Mr Albanese said some of the injured passengers are at John Hunter Hospital, but many have been flown to Sydney.
NSW Premier Chris Minns said the loss of so many lives was "nothing short of heartbreaking", adding: "For this horrific crash to have occurred on a day that should have been filled with love and happiness only adds to the heartbreak".
"For a day of joy to end in such devastating loss is cruel indeed. Our thoughts are also with those who have been injured," he said.
