"If you approach a ford in heavy rain: stop, go slowly, get out and do an assessment of how much water is in the ford before you drive through it," he said.
The Whitecliffs farmer has since pulled the car out of the Selwyn River with his tractor.
Grant said if people's vehicle is washed down a river and they are safe, they should contact police to make them aware no one is in danger.
It is not the only close-call to surface since the floods.
During the July floods two men in a four-wheel-drive were swept about 100m downstream in a wild torrent of water from a ford off Greendale Rd in Darfield.
The two men managed to get onto the roof of the vehicle and scramble up a pine tree before police, ambulance and fire services arrived, about midnight on July 23.
Grant said one of the men managed to get ashore by swimming through the river and grabbing onto the shrubs and bushes. The other stayed up the pine tree and was rescued by police.
One of the drivers used his cellphone to call a friend who alerted emergency services.