The bus rolled over at a roundabout near the New South Wales town of Greta in the Hunter region, also referred to as the Hunter Valley, about two and a half hours northwest of Sydney.
Authorities said initial reports suggest at least 10 people were killed and 25 hospitalized, with some flown to Sydney for medical care.
The bus driver, 58, was arrested, according to police. He was not injured in the incident, police said.
The bus had been carrying people returning from celebrations at a winery when it crashed, NSW state Premier Chris Minns said. Minns called the crash a “profoundly tragic event,” adding that residents of the region “will be shocked and saddened.”
Emergency reports came in just before midnight, with police dispatching multiple helicopters, firefighting units and ambulances to the scene.
NSW Police Acting Assistant Commissioner Tracy Chapman said it was too early to comment on the circumstances of the crash, but police had “sufficient information” to bring charges against the driver. Chapman did not specify what those charges were, but said the crash site is being treated as a crime scene.
Police are still trying to identify all the people on the bus but “as far as we’re aware there are no children involved,” Chapman said. As of Monday morning, the bus was still on its side; police said the bodies of those killed had not yet been removed, and it was possible more people could be discovered under the bus.
Australian Prime Minister Anthony Albanese expressed his condolences following the “horrific” crash.
“All Australians waking up to tragic news from the Hunter send our deepest sympathies to the loved ones of those killed in this horrific bus tragedy,” Albanese said on Twitter. “For a day of joy to end in such devastating loss is cruel indeed. Our thoughts are also with those who have been injured.”
In a statement Monday, the wedding venue said it was supporting guests who need their help, adding that it wished to protect their privacy.
The Hunter region is popular for weekend getaways and weddings. The crash also took place on a long weekend, with many tourists likely in the region ahead of a national public holiday on Monday.