Senior Sergeant Richard Whitmore, of Balclutha, said last night the crash happened at Ahuriri Flat, near Owaka.
Police, ambulance, fire service personnel and three helicopters were initially sent to the scene, after early reports the 12 occupants were trapped.
The Brooklands Village vehicle, which had been travelling in convoy with a second bus, had lost traction on the gravel road at low speed, causing it to roll 2m on to its side down a bank.
"Fortunately, the van had become entangled in a tree, which had prevented it from rolling further down a 20m bank into the paddock," Snr Sgt Whitmore said.
All 12 occupants were initially trapped, before being helped out of the van by members of the public and emergency services personnel.
All had been were wearing seatbelts.
They were taken to Balclutha Hospital and were later discharged.
