The bus driver charged over the deaths of 10 people in a horror crash in Australia’s Hunter Valley reportedly made a disturbing boast to passengers moments before losing control of the vehicle.

Fifty-eight-year-old Brett Button allegedly took to the microphone and told the wedding guests that packed his coach: “If you think that was fast … watch this” moments before the bus crashed at a roundabout in the town of Greta, the Daily Telegraph reported.

The newspaper said “multiple survivors” have told police and friends about the remarks and reported that other survivors told friends that the speed of the bus prompted them to tell others to get back into their seats before the crash.

Button will appear in court today after being charged with dangerous and negligent driving over the crash that shocked Australia, killing 10 and injuring 25 - the nation’s most deadly road accident in almost 30 years.

Police inspect underneath a bus that rolled onto its side near in the Hunter Valley. Photo / AP

Sunday’s crash happened just after 11.30pm in foggy conditions at the roundabout on Wine Country Drive in Greta, the gateway to the Hunter Valley region of New South Wales.

The guests had earlier attended a wedding at the Wandin Estate Winery and were heading for their accommodation in the town of Singleton, Police Assistant Commissioner Tracy Chapman said. One guest told Seven News it had been a nice day and a fairy-tale wedding.

A motorist who drove past the crash scene, identified by Australian Broadcasting Corp only as Alison, said the fog was so heavy she could not make out the colours of the flashing lights of police cars, ambulances and fire trucks.

“The fog was terrible,” she told the ABC. “It was super foggy. You could barely see in front of you.”

Of the 25 people injured, one was in critical condition and several others remained in hospitals, the state government said. The conditions of the others were described as stable.

Police had said 18 passengers escaped injury. But police later said there were only 36 people on the bus: the 10 dead, the 25 injured and the driver. The 18 were the least seriously injured among the passengers taken to hospitals.

Assistant Commissioner Tracey Chapman addresses the media in Huntlee near the scene of a bus crash in the Hunter Valley, north of Sydney, Australia. Photo / AP

Police Commissioner Karen Webb said investigators had not yet determined what caused the bus to roll on its side.

“The cause may not be known for some time. It will require scientific examination,” Webb told reporters.

Whether passengers were wearing seatbelts also “will come under scrutiny”, Webb said.

Chapman said rescuers smashed the front window to remove people from the wrecked bus.

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,” Albanese 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,” Albanese added.

- Additional reporting, AP