A wedding party bus crash killed 10 people in Australia's Hunter wine region north of Sydney, police said Monday, announcing the arrest of the 58-year-old driver.

The light-coloured coach lay on its side, television images showed, after overturning late Sunday night as it ferried guests from a wedding at a local winery.

A dozen emergency workers wearing high visibility yellow vests were at the scene, their vehicles' coloured lights flashing in the foggy night.

The toll may yet rise, with 25 passengers rushed to hospital -- two of them airlifted from the crash by helicopter, said Acting Assistant Commissioner Tracy Chapman.

The bus had yet to be righted with the deceased still inside, she said, leaving the possibility that others could be trapped underneath.

Police were not aware of any children being involved, she said.

The coach driver was subjected to mandatory testing at the hospital before being held at a police station in nearby Cessnock, the police officer told reporters at the scene.

"He's under arrest. He's been the driver of a motor vehicle collision where there have been fatal injuries and there will be charges pending," she said.

The passengers had been at a wedding together, the officer said, and were presumed to have been heading to their accommodation after the festivities.

No other vehicle appeared to have been involved in the crash, she said.

Cruel and sad

Prime Minister Anthony Albanese said weddings were among the happiest of times for friends and family.

"For a joyous day like that in a beautiful place to end with such terrible loss of life and injury is so cruel and sad, and so unfair," he said in Canberra.

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

Albanese sent his wishes for the recovery of those in hospital, and thanks to the emergency workers.

"I want to give our thanks to the first responders, those who arrived at this terrible scene. I cannot imagine what they were confronted with, what they had to deal with," he said.

The area is being examined by specialist forensic police and a Crash Investigation Unit.

The Hunter region is replete with vineyards, kangaroos and native bushland, making it a popular spot for tourists and group outings.

New South Wales Premier Chris Minns sent his "deepest condolences" to the families and loved ones of those killed and injured.

"This is a profoundly tragic event. I know the whole community of (New South Wales), and in particular the people of the Hunter, will be shocked and saddened to wake up to this news today," Minns said.

Investigations into the circumstances of the accident have commenced, led by New South Wales Police.