A Haast woman watched a tourist's rental car cross the centre line several times before taking matters into her own hands and confiscating his keys.
Sheri Wright at first thought the driver may have been drowsy.
But she became more and more concerned as she was behind the car for about 45 minutes on the road between Hari Hari and Franz Joseph on Monday.
"Just very very erratic driving and the serious concern was the crossing - every left hand turn he was crossing into the opposite lane."
Ms Wright was unable to call police because of cellphone blind spots so had decided to stop the car at the next town.
"He actually stopped in at his accommodation so we just pulled in behind him and basically said 'look you're driving dangerously I'm going to take your keys'."
He was agitated and did not appear to understand, so she took the keys from him.
The man's wife said they had just arrived in the country and did not know the rules.
Police later said there was enough evidence recorded on cellphone video footage to issue an infringement notice. She said the rental company had cancelled the agreement with the tourists in question and reclaimed the car.
It followed a similar incident in January when a member of the public had took keys off a driver in the Lindis Pass because of concerns about their driving.