Police are warning Christchurch vehicle owners to be vigilant, after vehicles have been stolen to knock people off bikes

Three youths have been arrested after a cyclist was seriously injured when she was thrown off her bicycle by a car.

Police are searching for a fourth person involved.

Three cyclists have reported they've been deliberately knocked off their bikes in Christchurch by motorists, in the last few days.

Christchurch Superintendent Lane Todd says the vehicles most commonly being stolen are Toyota Aqua, Nissan Tiida, Mazda Demio and Mazda Atenza.

He says owners of these vehicles should take extra precautions to keep their vehicles safe.