Fifteen people were taken to hospital after the single-vehicle crashes occurred 100 metres apart on the Tekapo-Twizel highway (State Highway 8) about 8.40am, police said.
Two people were seriously injured and taken by helicopters to Dunedin and Timaru hospitals. Eight people were in a moderate condition.
A spokesperson for China's Consulate-General in Christchurch said staff had visited the seriously injured pair and the embassy had asked travel agencies to accommodate the remainder of Chinese tourists involved.
Among the injured was a group who competed in the World Choir Games in Auckland last week.
Tony McClelland, who was travelling from Christchurch to Omarama this morning, said the road conditions were the worst he had ever seen.
He hit black ice close to Lake Tekapo Airport, nearly lost control of his van and almost ended up in a ditch.
He pulled over and called the police, asking them to close the road.
"You're looking -5, -4 [degrees Celsius], foggy conditions - that State Highway 8 should have been closed. No doubt and it wasn't, mate. There's just a big black sign up by the airfield saying 'dangerous conditions, black ice'. That's how people die," McClelland said.
Getting back on the road, he stuck to 60kmh and considered turning back.
Soon after, he saw two buses "flying" out of the fog.
"They were not doing 60[kmh], they were not doing 80. They were doing at least 100, probably a little bit more."
McClelland learnt that two buses had crashed once he arrived in Omarama.
The popular tourist road often had people tackling it who did not know how to drive to the winter conditions or what to expect, and would sit on 100kmh despite black ice in the area, he said.
Travellers' plea for better signs and awareness
Australian nurse Emily, who was driving to Christchurch when she saw the rolled buses, said she used a bystander's first aid kit to bandage two children who had blood gushing out of their foreheads.
"It was something you see out of a movie. Everyone was crying and people were bleeding out of their heads and arms," she said.
"We were just in shock so we all went over and tried to help where we could", including dampening cloths to try to clean wounds, keeping people warm and comforting the passengers who were in shock.
The bus had its windows smashed, there were debris and toys scattered around, and the emergency exit at the top had been popped open, she said.
She did not believe it was the driver's fault because the poor weather and road conditions meant he was "set up to fail".
Grace, who also was travelling along the road when she came upon the scene, said she too gave first aid to some of the shaken passengers for about 25 minutes before fire crews arrived.
"The children were quite lacerated by the glass, I believe," Grace said.
A man had helped to get all the passengers out of the first bus before she arrived.
"But as the temperature was about negative one (degrees), they were just freezing, so everyone was gathering all the blankets and scarves and jumpers that they could to give to these people," Grace said.
The driver appeared to be in shock with a broken arm or wrist, and the children were upset and crying, she said.
She felt shocked to not find more road safety signs and information along the highway.
"We were super scared on the road ourselves 'cos of the black ice and really poor conditions, so I think something really needs to be done about that.
"Considering it's a 100 [kmh] zone, we were going about 30 so I think that it's just not at all well equipped, especially for vulnerable tourists who aren't familiar with the conditions."
She wanted to see action so travellers were better informed about the road conditions and the highway was better maintained and able to cope with the conditions.
'Perfect storm of conditions for accidents'
NZ Transport Agency Waka Kotahi (NZTA) generally advises drivers to go slower than they normally would to reduce the risk of losing control or skidding when conditions are icy or snowy.
On Thursday morning, NZTA put out a black ice and winter driving conditions warning for that stretch of road.
Twizel Community Board member Tracey Gunn said she was not surprised to hear about the crashes near the Hayman Rd intersection, which was popular with tourists as it had a car park with toilets and breathtaking views.
"Close to the lake and with the fog, the roads are wet. Sub-zero temperatures means they freeze, black ice, and you can't see black ice. It just looks like a wet road," Gunn said.
"It's just a perfect storm of conditions for accidents, unfortunately."
The Serious Crash Unit is investigating. NZTA will also be examining the road and its condition as well as the two buses.
