Seven people have been injured - including two critically - after a two-vehicle crash involving a St John Health Shuttle south of Christchurch this morning.

Emergency services rushed to the crash on State Highway 1 near Burnham at 9.30am.

One of the vehicles has crashed into a tree, police say.

Six ambulances, four clinical managers, one rapid response vehicle and one helicopter responded, a St John spokeswoman said.

Seven patients have been transported to hospital, with two in critical condition, three in a serious condition and two with moderate injuries.

"There are multiple people involved and initial indications are there are serious injuries," police said.

Emergency services rushed to the crash on State Highway One near Burnham at around 9.30am. File photo / St John

One person was trapped in the first row of seats in the back of the van and had to be extracted by the Fire Service.

The passenger was treated by paramedics at the scene for two broken legs and open fractures to the lower limbs.

They were flown to Christchurch Hospital by the Canterbury Rescue Helicopter.

The northbound lane is blocked and traffic management is being put in place at the scene.

The St John Health Shuttle is a koha-based community service that transports people to health-related visits, and then brings them home again.