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

Police said the single-vehicle crashes occurred 100 metres apart on State Highway 8 - the Twizel-Tekapo Highway - about 8.40am this morning (Thursday July 18).

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, 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.

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

About 40 firefighters from four crews around Twizel, Lake Tekapo, Omarama and Otematata are at the scene.

Fire and Emergency said no one was believed to be trapped after the crashes, and they were not expecting more crews would be called for.

The road is closed between Tekapo and the intersection Lake Pukaki.

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.

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."