The incident happened near the intersection of State Highway 6 and Glen Nevis Station Rd.
Images from the scene show seven rescue helicopters along with emergency services surrounding a bus. People could be seen being transported away on stretchers.
A witness told the Herald a car had gone head-on into a tourism bus with a trailer.
”Seven rescue helicopters, six fire appliances, and maybe 10-15 ambulances at least,” he said.
”Major incident vehicles, support vehicles, managers, CIB police and road police.”
Hato Hone St John said it was notified of a motor vehicle incident on Kingston Rd around 4pm today.
Twenty people were treated with two critical, two serious and multiple people with moderate and minor injuries.
“Hato Hone St John responded [with] nine ambulances, two operations manager, one rapid response unit, one major incident support team, and seven helicopters,” it said on its alerts site.
The New Zealand Transport Agency said SH6 north of Kingston township is now closed due to the crash.
“Police serious crash investigations will be required following this incident,” NZTA said in a post online.
“It is highly likely the road will remain closed for several hours, possibly late into this evening.”
SH6 KINGSTON, OTAGO - SERIOUS CRASH - UPDATE 8:30PM
State Highway 6 remains CLOSED between Kingston township and Wye...
“It honestly sounds like a major crash, because they said there was a person in the car the bus hit … I reckon the road will be closed for some time,” he said.
Another local business owner believed the road would remain closed overnight.
”I think the bus that was involved in the crash is blocking the road.”
Motorists are urged to delay travel where possible.
An alternative route south is available via SH6, SH8B, SH8 t, SH90 to SH94 and back onto SH6. The reverse is in place for northbound travel. Allow an additional 2 1/2 hrs for this diversion.