At least 10 people died and 25 were injured after a chartered bus carrying wedding guests rolled off a ramp at a roundabout in Australia's New South Wales (NSW) state, police said on Monday.

The accident occurred around 11:30 p.m. (1330 GMT) on Sunday near the town of Greta about 180 km (112 miles) northwest of Sydney, in the Hunter region famous for its vineyards and wedding spots.

"I understand they had been at a wedding together, it's my understanding they were travelling together ... presumably for their accommodation," NSW Police Acting Assistant Commissioner Tracy Chapman said during a televised media briefing.

At this stage, it appeared to be a single-vehicle accident, Chapman said. Police were still trying to identify all the passengers, she said.

Footage shown on Australian media showed the bus lying on its side. Some people could be still trapped beneath the vehicle, police said.

The driver of the bus, a 58-year-old man, was under arrest and was expected to be charged over the accident, Chapman said. He had been taken to hospital for mandatory alcohol and drug testing.

Heavy fog was present in the area at the time but the cause of the road accident - the worst in Australia in nearly 30 years - had not been determined, Chapman said.

The two worst bus accidents in the country were head-on collisions within two months of each other in 1989 that killed 35 and 21 people, both in NSW state. Eighteen people died in 1973 when a tourist bus plunged down a slope after a brake failure.

'CRUEL, SAD AND UNFAIR'

Prime Minister Anthony Albanese expressed his "deepest sympathies" to the families of the people killed and injured.

"All of us know the joy of going to a wedding ... they are some of the happiest times that you can have. For a joyous day like that in a beautiful place to end with such terrible loss of life and injury is so cruel and so sad and so unfair," Albanese told reporters.

Residents of the area gathered to pay tributes and lay flowers near the scene of the accident.

"When we heard about it this morning, it was like, Oh my god! It's way too close to home. And a wedding, one of the happiest days of your life and turns into one of the most tragic days of your lives," Kim Greko told television channels.

The Wandin Valley Estate winery which hosted the wedding was closed on Monday, a public holiday, the Australian Broadcasting Corp reported.

"We are deeply saddened to hear of the bus crash overnight that has claimed the lives of some of our guests," ABC reported, citing a statement from the vineyard.