A crash in Waikato last night that saw five children thrown from a car - and badly injured - is now under police investigation

The crash involving one car on Morrinsville-Walton Road in Matamata was alerted to police just before midnight.

Five children aged between ten and 13 were found seriously injured outside the car, in a ditch.

Four were taken to Waikato Hospital by ambulance, and one by helicopter.

Four of the children remain in a critical condition and another is stable.

There's nothing to indicate anyone else was in the vehicle.