Mr Macdonald said it was "surprisingly easy" to kick off the clamp.
He said the company claimed he had parked in an area of the company's Rialto building reserved for disabled drivers, but he had photographic evidence to the contrary, showing he was in a space covered by his monthly fee.
Despite that, he acknowledged part of his car might have been "a few centimetres" outside his general parking space.
"I think I had one wheel over the edge of the line, and that's when they put the clamp on."
But Mr Macdonald considers the company unduly punitive.
"I cannot believe the way they have treated a long-term customer who has not missed a payment for 18 years."
A parking officer said nobody would be available to comment on Mr Macdonald's case until after the weekend.
He could not explain why Mr Macdonald's car was immobilised, but confirmed $300 was the standard fee for damaging a clamp.