Eight people who were stranded on top of two cars stuck in the Waimakariri River have been pulled to safety.

Police were called to the scene in Canterbury just after 4pm, with reports two cars were stuck in the river and their occupants had escaped to the car roofs.

Canterbury Coastguard services had to be called in to help, because the water was reportedly moving quickly.

St John ambulance crews also attended the incident, though it was unclear if there were patients needing medical help.

South Island region spokesman Gerard Campbell said it wasn't clear how the cars ended up in the river.

It's believed the vehicles were caught in a flash flood which saw the river level rise dramatically in a very short time.