It was the fastest speed the veteran of more than 30 years had recorded. The previous highest was 196 km/h and he also recalled a few drivers in the 170km/h range.
Mr Doonan said Saturday's incident happened on a flat section of a regional road where he often parked with the speed gun.
The driver pulled over, Mr Doonan said. There were two passengers in the car, but neither had the correct licence to take the wheel.
So he followed him home and handed him a 28-day suspension of his driving licence.
A 21-year-old local man will appear in the Timaru District Court next week on one charge of driving at a dangerous speed. He faces a minimum six-month ban from getting behind the wheel.