Emergency services were called to Walnut Ave, Ashburton, near the roundabout with West St, after the incident about 12.20pm.
Two ambulances attended the scene.
One person was treated at the scene and then transported to Ashburton Hospital in a serious condition.
A police spokesman said they were alerted to a vehicle collision at the intersection.
"The crash involved a car and pedestrian," he said.
"The driver is assisting police with enquires."