A woman has been flown to hospital in a serious condition after her car crashed into a ditch north of Auckland.

The crash, near Glorit on the Kaipara Coast Highway north of Helensville late Monday afternoon, trapped the injured motorist, who had to be cut from the wrecked vehicle.

Firefighters spent 40 minutes freeing her before she was flown by rescue helicopter to Auckland City Hospital.

The woman, in her 50s, was the only person in the vehicle.