At least 10 people are feared dead after a late-night bus crash in the Australian state of New South Wales on Sunday, local police say.

The driver of the bus was arrested after the bus rolled over at a roundabout near the town of Greta, which is located in the popular wine growing Hunter region, the New South Wales Police Force said.

Acting Assistant Commissioner Tracy Chapman said during a news conference that initial reports suggest 10 people were killed in the crash and the number of people hospitalized had increased from 11 to 25.

Emergency crews responded just before midnight to reports about the bus rolling over. Police said multiple helicopters, highway patrol, as well as fire and ambulance responded to the crash.

The 58-year-old bus driver was taken to a hospital for mandatory testing and assessment after the crash, Chapman said, adding he was not injured.

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 detail what charges the driver would face but said the crash site was being treated as a crime scene.

Police were still trying to identify all the people on the bus but "as far as we're aware there are no children involved," Chapman said.

Chris Minns, the Premier of New South Wales, said the bus was returning from a wedding at a local winery when it crashed.

"This is a profoundly tragic event," he said. "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."

The Hunter region -- also referred to as the Hunter Valley -- is about two and a half hours northwest of Sydney. It is one of Australia's leading wine regions and popular for weekend getaways and weddings.

The bus was still on its side Monday morning. Police told reporters that the bodies of those killed had not yet been removed from the site and it was possible more people could be discovered under the bus when it is moved.

Australian Prime Minister Anthony Albanese tweeted his condolences to those affected by the 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. For a day of joy to end in such devastating loss is cruel indeed. Our thoughts are also with those who have been injured," Albanese said in the post.

He also tweeted his thanks to the first responders saying, "Thank you to all the first responders who rushed to the scene, and those continuing to assist and care for those affected by this tragedy."