Four people have been taken to hospital after two bus crashes on a State Highway 8 in the Mackenzie District.

Police said the single-vehicle crashes occurred 100 metres apart on State Highway 8 - the Tekapo-Twizel Highway - about 8.40am on Thursday.

Hato Hone St John said it was notified of the incidents at 8.38am and 10 vehicles and a helicopter had been dispatched.

Four patients have been hospitalised; two in serious condition and one in moderate condition and another with minor injuries.

The two seriously injured had been taken to Dunedin and Timaru hospitals via helicopter.

Police said it was understood those on board were international tourists, and the relevant Consulate had been advised.

A Chinese Embassy spokesperson said staff from the Christchurch Consulate was on the way to the scene.

Passengers on board the buses who have not required medical attention have been taken to Twizel.

Earlier, about 40 firefighters from four crews around Twizel, Lake Tekapo, Omarama and Otematata were at the scene.

Fire and Emergency (FENZ) said no one was believed to be trapped after the crashes.

The road is closed between Tekapo and the intersection with State Highway 30.

The New Zealand Transport Agency / Waka Kotahi (NZTA) advised motorists to expect delays, with this section of the road possibly staying closed for the rest of the day and no immediate detours available.

"Drivers should allow at least two hours longer for travel times while SH8 remains closed.

"Westbound travellers should use SH8 east to Timaru, SH1 south to the SH82 junction, and the inland route on SH82 to Kurow/SH83 junction, with westbound to SH8/83 Otematata. Eastbound travellers should use the reverse route."

The Serious Crash Unit is investigating.

"Weather conditions were foggy at the time, however the exact cause of the crashes is still being determined," police said.

A FENZ shift manager said crews were faced with poor weather, fog, ice and multiple patients.

"Crews worked extremely hard in trying conditions to get the best outcome for the public," he said.

Fire and Emergency crews left the scene about midday, but the road remained closed.

NZTA said as this was a serious crash on a state highway, they would be conducting an investigation of the road and roadside conditions, alongside police investigations.

"The condition of both buses will also be examined as part of the investigation.

"Our thoughts are with everyone affected by this morning's crash."

Intersection near bus crashes popular with tourists - board member

Twizel Community Board member Tracey Gunn said conditions in the area could be treacherous, making locals wary.

She said she was not surprised to hear about the two bus crashes near the Hayman Road 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."

On Thursday morning, NZTA put out a black ice and winter driving conditions warning for that stretch of road.

Gunn said it was one of the two intersections in the area which had the most crashes, because it was a sweeping bend which was a bit tricky to go through even in good conditions.

"If the visibility is very low and so if you're driving down the main road, you wouldn't even know that there was potentially an intersection there."

Under winter conditions, motorists needed to go slower, brake on a straight, not on a corner and factor in more travel time, she said.

Main image: Supplied.