The passengers were returning from a wedding at a winery on Sunday night in Hunter Valley, a popular spot for wine tourists, when their coach overturned.
Police have charged the 58-year-old bus driver with 10 counts of dangerous driving which resulted in death.
They said they were still in the process of identifying the dead.
The newly-weds, however, weren’t reported to be on the bus.
Police commissioner Karen Webb said the site of the crash is “still an active crime scene”. “We’ve got forensics officers processing the crime scene, we’ve got crash investigation unit officers, we’ve got rescue officers [on scene],” she added.
The accident occurred about 23:30 local time [13:30 GMT] when, according to police, there had been heavy fog in the area. The bus had rolled over while making a turn at a roundabout off a highway. Authorities say the vehicle has now been pulled upright.
NSW Police Acting Assistant Commissioner Tracy Chapman said the guests were travelling to Singleton “presumably for their accommodation”. Two of the survivors were airlifted from the crash, she added. Local media report that at least one of them is still in a critical condition.
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.
Hunter Valley in New South Wales is known for its vineyards and native bushland, making it a popular spot for wine lovers and group outings or celebrations.
A guest at the wedding said the day had been a “fairy tale” until news of the accident broke.
“We all started panicking,” he told 7 News.
Police said they are still working to identify the crash victims and contact their next of kin.
“Family and friends of a person who may have been on board the bus are urged to contact Cessnock Police Station,” they said in a statement.
