Emergency services were called to the scene, on the Kilmog, at 10.15am.
A police spokeswoman said a car and grass were on fire, and one lane of the highway had been partially blocked.
The sole occupant of the vehicle was out of the car and was uninjured.
A Fire and Emergency New Zealand spokeswoman said crews from Waikouaiti and Waitati were at the scene.
Waitati Volunteer Fire Brigade station officer Richard Templeton said it appeared a mechanical fault was behind the fire.
Firefighters were called out at 10.20am and were back at the station by 11.15am, he said.