Two cruise passengers from America remain in Christchurch Hospital after a horror bus crash on Akaroa peninsula.

Today police are still at the serious smash on the Christchurch Akaroa Rd, although the highway was reopened to traffic around midnight.

The Guardian is reporting the most seriously injured of the 31 passengers were from the United States.

The bus, which was transporting passengers back to the Celebrity Solstice cruise ship, collided with a car before plunging down a steep bank yesterday afternoon.

The newspaper said six guests from the United States were among the injured along with the drivers of both vehicles.

The remaining passengers had returned to the ship, it said. The ship has now arrived at Dunedin.

Four tourists had been discharged from hospital overnight.

Meanwhile the cruise company had sent members of its care team to Christchurch to offer immediate support and assistance to the injured passengers.

"All other guests have returned to the ship. Our thoughts and prayers are with all those involved and we hope for their speedy recovery," said the company.

Police said the bus and the car collided just after 5pm and the bus ended up down a bank near the intersection of Summit Rd and the Christchurch Akaroa Rd.

Those with the worst injuries were flown to hospital by rescue helicopter.

This morning a New Zealand woman driving the car remained in a critical condition.

One man travelling in the bus was in a serious but stable condition and two female passengers were stable.

Yesterday's crash left two with critical injuries, a further five suffering from serious injuries and one person with moderate injuries.

Another 26 people, who either had minor or no injuries, were taken to Akaroa Fire Station on another bus for paramedics to assess them before rejoining the cruise liner in Akaroa Harbour.

Camera operator Matthew Simons said the bus was about 50m down the bank and a blue wagon-like Toyota on the road.

Debris from the smash was strewn on the road and the front of the car was mangled.

Pieces of the car were lined up on the side of the road, resting on a bank.

Tragedy has struck Celebrity Solstice on New Zealand roads three times in the past few months.

A Canadian passenger from the ship was severely injured when he was hit by a campervan while cycling with his partner on Otago Peninsula nearly two weeks ago.

And an elderly American woman who was on board in November was killed while riding a bike in Tauranga the day the ship arrived in the city.

The same afternoon a bus carrying passengers from the same vessel collided with a car on State Highway 29. The car's driver died, but nobody on the bus was injured.