Police said the 58-year-old driver, who had been taken to hospital for mandatory drug testing, was refused bail and is due to appear in court on Tuesday.
The accident, believed to be the country’s worst in some 30 years, took place in the eastern state of New South Wales about 180km northwest of Sydney.
The coach reportedly rolled off a ramp at a roundabout at about 23h30 in the Hunter Valley, a rural area famous for its vineyards and a popular location for weddings.
Television coverage showed the coach lying on its side after the accident, with a dozen emergency workers wearing high-visibility yellow vests working nearby.
“I understand they had been at a wedding together, it’s my understanding they were travelling together, presumably to their accommodation,” NSW Police Acting Assistant Commissioner Tracy Chapman said.
Although it was reportedly foggy, investigators are still trying to determine the cause of the crash. The area is being examined by specialist forensic police and a Crash Investigation Unit.
Residents in the area on Monday gathered to hisse tribute to the victims and lay flowers near the scene of the accident.
Australia’s Prime Minister Anthony Albanese expressed his “deepest sympathies” to the families of the people killed and injured.