A horror Christmas Day smash near Kaikoura has left five people seriously injured.

It's understood two vehicles collided on State Highway 1 at the Boat Harbour Camping Ground, not long after 6PM.

Police aren't confirming details of the incident, but an eyewitness at the scene has told our newsroom it looks like one car crossed the centreline.

She says she heard a loud bang from the campground.

Two have been airlifted to hospital, and another three taken away by ambulance.

The witness says the car which apparently crossed the centreline, appeared to be driven by European tourists.