Emergency services were called to the horrific scene at Piarere on State Highway 1 at 2pm on Wednesday after a bus load of foreign tourists and two other vehicles collided on a northbound passing lane.

The DNZ Travel bus was carrying 11 tourists and a driver.

Police said three other people were spread across two other vehicles, a black Mazda ute and a silver Mazda liftback car.

The driver's side of the bus was mangled and the impact caused major damage to the fronts of the other two vehicles, with debris strewn across the highway near the intersection with Paparamu Road.

One person died at the scene, two people were critically injured, and four others suffered serious injuries.

Hato Hone St John said 13 injured people were transported by road and air to Waikato, Rotorua, Tauranga and Auckland hospitals.

A Northern Rescue spokesperson said both Auckland Westpac Rescue Helicopters airlifted a man in his 40s and a teenager to Auckland City Hospital.

Photo: Auckland Rescue Helicopter Trust

Five rescue helicopters were dispatched to the scene including two from Auckland, one from Taupō, and one from Waikato.

They joined the Eastland Rescue Helicopter from Tai Rāwhiti which was first to arrive after being diverted mid-return flight from Hamilton.

A spokesperson said the Eastland rescue chopper said it flew a patient in critical condition to Waikato Hospital.

Another two patients were also flown to Hamilton.

In total, St John responded seven ambulances and rapid response units.

Fire and Emergency, police and the Serious Crash Unit were also at the scene and the road was closed until 9pm to enable a scene examination.

Taupō road policing manager senior sergeant Fane Troy said several of the injured had been discharged from hospital on Thursday morning and police were collecting statements from them along with evidence to establish the cause of the crash.

"Early indications are that we have had a vehicle cross the centreline. The reason for that is uncertain. That is part of our investigation, to determine what that has happened."

Taupō road policing manager senior sergeant Fane Troy. Photo: RNZ / Natalie Akoorie

Troy earlier said the straight stretch of road was not a blackspot. The weather conditions were also fine and clear.

Troy said police's thoughts went out to those affected by this tragedy and he reminded motorists to take their time and be courteous to other road users, and safe when driving.

He said police would not be in a position to release the deceased's name until next-of-kin were notified.

DNZ Travel was a luxury tour company in New Zealand aimed at foreign travellers, according to its website, which was partly written in simplified Chinese.

It promotes custom itineraries including visits to Rotorua, Hobbiton and Waitomo Caves in the Waikato and Bay of Plenty regions and says it is not a rental car company.

"All our vehicles are driven by our experienced drivers with their passenger service endorsement."

A person who answered the phone at DNZ Travel confirmed the tourist group was okay but declined to comment further and deferred questions to police.

The Chinese Embassy said its initial enquiries found the tourists were Korean while the driver's nationality was still being confirmed.

Yesterday's crash was the second in six months in the Waikato region involving a passing lane where oncoming traffic was not separated by a median barrier.

The first was in Te Awamutu in May when five people died after two cars collided on State Highway 3.

In that case a vehicle also crossed the centreline.

- Additional reporting by Liu Chen

-RNZ