Emergency services were called to the crash site about 11.30pm on Sunday at a roundabout on Wine Country Drive near Greta after reports a coach had rolled.
Initial inquiries indicate 10 people have died, 11 were injured and have been transported to hospital via air and road ambulances, and a further 18 passengers were uninjured, according to a police statement.
An investigation into the circumstances surrounding the crash has commenced, with a large-scale emergency response involving local police, rescue squad and highway patrol officers, ambulance paramedics, NSW Fire and Rescue and the Rural Fire Service.
A crime scene has been established, which will be examined by specialist forensic police and the force's Crash Investigation Unit.
The 58-year-old driver of the bus has been taken to hospital under police guard for mandatory testing and assessment.
Wine Country Drive remains closed in both directions at Greta, west of Maitland.
Motorists are being advised they can divert around the closure via Bridge Street, Drinan Street, Clift Street and the New England Highway.
The northbound and southbound off-ramps from the Hunter Expressway to Wine Country Drive are also closed, although through lanes are not affected.
Due to the nature of the crash, the NSW Transport Management Centre says there is no forecast for how long the road will be closed.
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