The 58-year-old bus driver was taken to hospital for mandatory testing and subsequently arrested with charges expected.
Emergency services were called to the crash site about 11.30pm on Sunday at a roundabout on Wine Country Drive near Greta after reports the coach had rolled.
A total of 10 people were confirmed dead while a further 25 were taken to hospital with varying injuries, including one person who remains in a critical condition.
The bus was returning guests from a wedding reception at Wandin Estate winery in Lovedale to Singleton, roughly 30 minutes away, where several media outlets have reported the group is from.
As of 10am on Monday, 21 people remained in hospital, according to a spokeswoman for NSW Premier Chris Minns.
One person is in a critical condition at John Hunter Hospital, where eight others are stable, while five patients are in a stable condition at Maitland Hospital, five are stable at Calvary Mater Newcastle and two patients are in a stable condition at Sydney's Royal Prince Alfred Hospital.
Police Acting Assistant Commissioner Tracy Chapman said it appeared only one vehicle was involved in the crash and that there was "sufficient information" to charge the driver, who was not seriously injured and taken to Cessnock Police Station.
"He's been the driver of a motor vehicle collision where there have been fatal injuries and there will be charges pending," she said.
As of Monday morning the bus remained on its side and Ms Chapman said it was possible the death toll would increase if more people were found trapped under the bus or if those with injuries worsened.
Ms Chapman said she understands a number of people who survived the crash were able to free themselves and walk to get help.
Singleton Mayor Sue Moore said the close-knit community and surrounds would be deeply affected by the event.
"If anything could be worse - people attending a wedding, and then on their way home from the wedding. It is just so tragic," she said.
"It will go on for weeks and months trying to heal from this."
An investigation into the crash has started, following a large-scale emergency response involving local police, rescue squad and highway patrol officers, ambulance paramedics, NSW Fire and Rescue, and the Rural Fire Service.
The bus was owned by Linq which provides school bus and event charters, with a fleet of buses which according to its website are all fitted with seat belts.
A crime scene has been established, which will be examined by specialist forensic police and the force's Crash Investigation Unit.
No children are believed to have been involved in the crash, according to Ms Chapman.
Mr Minns expressed his condolences to those impacted by the tragedy and said support is being provided.
"I know the whole community of NSW, and in particular the people of the Hunter, will be shocked and saddened to wake up to this news today," he said.
Prime Minister Anthony Albanese told reporters in Canberra for a day of joy in a beautiful place to end in such devastating loss was cruel and unfair.
"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.
"The mental scars of this will not go away.
"They are there forever and we need to make sure when the spotlight leaves, the care doesn't, for people who have witnessed this and poor people who've experienced this tragedy first-hand."
Wandin Estate said it is providing support to guests who need it and wished to maintain the family's privacy.
"We are deeply saddened to hear of the bus crash overnight that has claimed the lives of some of our guests," its statement said.
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