The 58-year-old bus driver was taken to hospital for mandatory testing and 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 in the Hunter Valley after reports the coach had rolled.
Ten people were confirmed dead and at least 21 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, about 30 minutes away, where several media outlets have reported the group is from.
Media identified the newly-wed couple as Mitchell Gaffney and Madeleine Edsell, who are highly involved with local AFL club the Singleton Roosters.
By mid-morning 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.
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 the bus was involved in the crash.
Ms Chapman said there was sufficient information to charge the driver, who was not seriously injured and was taken to Cessnock Police Station.
"He's been the driver in a motor vehicle collision where there have been fatal injuries and there will be charges pending."
The bus remained on its side on Monday.
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.
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 would be deeply affected by the tragedy.
"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.
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, Ms Chapman said.
Mr Minns said support was being provided to people affected by the tragedy.
"This has been a terrible, terrible event in the history of NSW - and I think today is a day to hug someone that you love and remember how precious life is," he said.
For a day of joy in a beautiful place to end in such devastating loss was cruel and unfair, Prime Minister Anthony Albanese said.
"People hire a bus for weddings in order to keep their guests safe and that just adds to the unimaginable nature of this tragedy," Mr Albanese said in Canberra.
"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 was providing support to guests who needed 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," it said in a statement.
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