"Do [they] want a list of my injuries stuck to my car? It's none of their business."
His list was long.
Among other ailments, Walker has had a stroke, spinal surgery, hand and elbow operations, double knee surgery on one leg, and is waiting for a full knee replacement on the other. He also has a pacemaker.
But these health issues were deeply personal, he said.
Although he only used the parking space when his heart was giving him trouble or he felt his knees would give way, the abuse from strangers made him feel embarrassed to do so.
Walker even swapped vehicles with his wife to avoid additional verbal attacks that were made worse when driving his sports car.
But it was other people he worried about.
The former canine instructor moved to Oamaru three years ago, but said this happened everywhere, and not just to him.
According to the Office for Disability Issues, one in four New Zealanders has a physical, sensory, learning or mental health disability.
And they were not all visible – as was the case for Walker.
The army veteran could stand up for himself, but there were others who could not, he said.
He understood people might be trying to discourage those without disabilities from using the parking spaces, but that was no excuse for abuse.
"Just because they can't see things . . . people are pretty closed-minded," he said.
But the 73-year-old did not want people to feel bad for him – he said he had a beautiful wife who kept him young and he woke up happy every day.
"There are a lot of people worse off than me.
"I have both legs and both arms. I'm lucky to be alive."
He just wanted people to have more empathy and compassion for others.
"If a person has a mobility card, they obviously have health issues.
"Give them a bit of room to go along their daily routine without being persecuted."