He called 111. Station officer Brent Key, of Willowbank Station, said firefighters discovered the car was unlocked and gave the dog water to drink while airing out the "hot" car.
Soon after, the "upset" dog owner showed up and told the crew he thought he had left a window down.
Mr Key said if the car had been locked, the firefighters would have called the SPCA, and if an inspector had confirmed the dog was distressed, they would have smashed a window.
SPCA Otago executive officer Sophie McSkimming said if the temperature was 30C outside, the temperature inside a car parked in the shade, with the windows slightly open, would be 39C within 10 minutes.
"In half an hour, it will go up another 10C."
On a hot day, dogs should be left at home rather than being left in a parked car, even for a few minutes.
Anyone seeing a heat-distressed dog in a parked car, should phone 0800 682 467.