It happened just after 11.30pm local time on Sunday in foggy conditions in the town of Greta, New South Wales - about 112 miles (180km) northwest of Sydney.

The bus driver, a 58-year-old man, was arrested after mandatory hospital tests.

A police spokeswoman told reporters that there was "sufficient information for us to establish there will be charges" taken against the driver.

The bus reportedly "rolled" off a ramp at a roundabout, with two of the injured being airlifted to hospital.

It is understood 18 passengers were unhurt.

Tracy Chapman of New South Wales Police said it was a "possibility" that the number of people killed may rise, as more passengers may be trapped underneath the overturned bus.

Ms Chapman said the crash site was "a very traumatic scene" - adding "wherever you have that large-scale incident with that many people, it's going to be difficult for anyone".

She also added that it is understood there were no children involved in the crash.

The bus had been taking about 50 guests back from a wedding to their accommodation at the time.

Family and friends arrived at the crash site on Monday morning - but police say they are still trying to identify everyone who was on the bus.

One wedding guest told reporters: "It was a nice day, a pretty good wedding ... fairytale stuff really ... we got the news that there had been a crash, and we all started panicking."

The guests had earlier attended the ceremony at the Wandin Estate Winery.

'It will send shockwaves'

Jay Suvaal, the mayor of nearby Cessnock, said the crash was "truly horrific".

He added: "We are a major wedding and tourist destination in the Hunter Valley.

"So there will be people from all over the state and the country that have been to these areas and have probably done similar things.

"I think it will send shockwaves right through the broader community."

The wedding was in the middle of a long weekend, with Monday a public holiday across most Australian states.

Australian Prime Minister Anthony Albanese thanked first responders and offered government support to victims and their families, saying the "mental scars of this will not go away".

"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," he told reporters.

"People hire a bus for weddings in order to keep their guests safe. And that just adds to the unimaginable nature of this tragedy."

Greta is in the heart of the Hunter Valley wine region - a picturesque area dotted with vineyards and restaurants. It was the first wine region established in Australia.