All 17 passengers on a tourist bus which rolled on to its side in icy conditions near Te Anau have been transported from the scene and police are investigating the cause of the crash.
St John treated 15 people, two with serious injuries, after a bus crashed on a "lethal" stretch of State Highway 94 between Mossburn and Te Anau this morning.
Emergency services were called to the crash at 8.10am, where the bus rolled on its side at the bottom of Gorge Hill near The Key.
It is understood the passengers were heading to Milford Sound for a cruise.
St John spokesman Gerard Campbell said four patients were transported by helicopter to Dunedin Hospital, two with serious injuries and two with moderate injuries.
Another two patients were being transported from Fiordland Medical Centre in Te Anau by helicopter to Southland Hospital with moderate injuries.
Police said the rest of the hurt passengers all received minor to moderate injuries and were transferred by bus to the Fiordland Medical Centre.
The ambulance service earlier said 19 people were treated, but this had since been updated.
Hamish McClean, who owns the farm where the bus crashed, said the vehicle did a 180- degree turn before rolling and sliding into his fence.
"It's people just not driving to the conditions - I could have put my hat on it that it was going to happen this morning."
The road was earlier closed, but the westbound lane was now open and stop-go traffic management was in place.
The New Zealand Transport Agency said the road was clear and fully reopened at 2pm.
McLean said a rental car had also slid off the road this morning about 1km from where the bus crashed.
Today was the third frosty day in a row and there was a dangerous combination of frost mixed with fog.
"Nine times out of 10 the accidents happen on a straight road when speed becomes a factor.
"The road will closed for a good hour or two I'd say."
McLean praised the rescue helicopter crews that arrived to treat the injured bus passengers, saying they had done an amazing job, However he believed local helicopter crews were needed, who knew the terrain and could get there faster.
A photograph of the crash scene supplied to the Otago Daily Times showed a white tourist bus, which was understood to be heading towards Milford Sound, on its side.
There were also signs of a heavy frost, with grass on the side of the road white.
Fire and Emergency New Zealand southern communications acting shift manager Ian Littlejohn said three people had to be cut free from the bus.
NZTA earlier warned road users about black ice on the stretch of road.
SH94 LUMSDEN TO TE ANAU - CRASH - ROAD CLOSED - 9:00 AM THURS 31MAY
— Waka Kotahi Otago & Southland (@WakaKotahiOS) May 30, 2018
Due to a crash on SH94 Te Anau - Mossburn Hwy near Mavora Lanes Rd intersection the road is currently CLOSED. Motorists are advised to expect delays. ^JPhttps://t.co/kM75nsfDZN
The Commercial Vehicle Safety Team and the Serious Crash Unit was investigating.
A woman who runs a nearby holiday home said she had not seen the crash but the highway was icy, especially in shaded areas, and it was also foggy.
"That bloody gorge it's lethal," she said of the crash site.
Te Anau chief fire officer Graeme Moffat said Te Anau was blanketed in fog and a black ice warning was in place this morning.
"You generally get black ice around the edge of the fog and there's a few shaded spots in the gorge."