A man was charged with dangerous driving on Monday after at least 10 wedding guests were killed when the bus they were travelling in crashed at a roundabout in Australia's worst bus accident in almost 30 years, police said.

Twenty-five people were injured in the accident around 11:30 p.m. (1330 GMT) on Sunday near the town of Greta, about 180 km (112 miles) northwest of Sydney, police said.

The bus carrying 35 passengers left the road and flipped onto its side at a roundabout after a wedding in the Hunter region, a rural area famous for its vineyards and wedding spots.

Police said the driver, a 58-year-old man, had been taken to hospital for mandatory drug testing. He was refused bail and is scheduled to appear in court on Tuesday.

Australian 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.

NSW Police Commissioner Karen Webb said police were treating it as a single-vehicle accident and the cause may not be known for some time.

The bus had been pulled upright after a "delicate operation", given dead passengers were inside, she said.

'Too close to home'

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.

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 reporters.

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.