Emergency services are at the scene of a serious three-car pile-up near Geraldine, south west of Christchurch.

The three-car collision occurred on the Geraldine-Fairlie Highway shortly before 10am, Senior Sergeant Richard Chambers said.

One person has been seriously injured and a rescue helicopter is on the way to the scene. It is understood some of the occupants of the vehicles were trapped.

Police are advising motorists to avoid the area as both lanes are blocked.

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"Icy roads are also making driving conditions hazardous throughout the south so, if you are driving, take extra care."