Although "it didn't really hurt at all", the man said they exchanged further words when both were in the bank a few moments later.
However once the motorist completed his transaction "he just turned around and gave me a good old whack ... a swinging right arm across the face", the man said.
"I was just taken aback that anyone would do that in a bank."
Having noted the plate number of the motorist's vehicle, the man later laid a complaint with the police.
Police told him the vehicle was registered to the former National MP. The man, who was not previously familiar with the former MP, identified him as the alleged attacker from a photograph.
The former MP did not return the Herald's calls or messages this week and was not at home when the Herald visited his house.