A bus carrying wedding guests overturned and killed 10 people in an "unimaginable" night-time crash in a popular wine region near Sydney, police said Monday -- Australia's deadliest road accident in 16 years.

The white coach was ferrying guests from a wedding at a local winery when it flipped onto its left side late on Sunday, coming to rest on the pavement alongside roadside barriers at the entrance to a major roundabout.

A dozen emergency workers wearing high visibility yellow vests were at the scene on the foggy night, helping to ferry more than 20 people to hospital with their vehicles' lights flashing blue and orange.

One patient was in a "critical" condition and 20 others "stable", police said.

Wedding photos shared on social media showed smiling guests from the party gathered on the lawn of a local winery in celebration, just hours before the accident -- the deadliest on official records since 2007.

The 58-year-old bus driver was given "mandatory testing" at a hospital before being arrested and held in custody at nearby Cessnock police station, Acting Assistant Commissioner Tracy Chapman told reporters.

He is cooperating with the investigation and "at this stage no charges have been laid," she said. 

"We are investigating all avenues of driver behaviour."

Some passengers in the crash "were able to free themselves and walk to help and aid", Chapman said, and no children are believed to be involved.

The passengers were presumed to have been heading to their accommodation after the festivities, Chapman said.

No other vehicles 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 Valley 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.

Wedding guests had come together to experience a "day of joy" and had instead had been met with "undeniable despair", he told reporters.

"The next few days and the next few weeks may be worse than the initial shock as it fully comes to the realisation of what this community has gone through."

djw/arb/mtp