He said Ryan became impatient and refused to serve him.
Security camera footage then showed the two attempting to enter the service station's door.
With Ryan slightly ahead, Stone attempted to get past, the younger man spun round and king hit the vet in the right eye.
The punch, which sent him stumbling backwards, fractured his skull and eye socket.
He required a metal plate to be inserted into his face and still needs surgery.
Once the footage was shown to the court, Ryan's lawyer Nigel Hewat asked for a short adjournment before returning to change his client's plea to guilty.
Judge Connell said the attendant had no self-defence argument.
"He touched you from behind and you instantly reacted with this right arm hook to the face," the judge said.
He sentenced Ryan to 80 hours' community work.
"You need to go carefully ... before you act like this ever again, think of the consequences," the judge said.
Less than an hour later, Stone appeared in the dock to plead guilty to charges of assault with a weapon and possession of an offensive weapon.
Court summaries showed after being punched he went back to his vehicle to fetch a machete from the boot and returned to the counter, where he fought with Ryan and cut his arm and torso.
Stone's lawyer Phil Jensen said the attendant's injuries were far less significant than his client's, and that the cut to the arm was accidental.
Judge Connell disagreed and said taking a machete into a confrontation was "reckless, not accidental".
Judge Connell said blame lay squarely in both camps.
"Your behaviour was as bad as his," the judge said. "Afterwards the stupidity of it [machete attack] must have come home to you.
"The serious aspect of this is that the machete was used, which I think puts you in a slightly different position to the other half of this equation.
"There is an element of provocation to some extent, but the law does not allow you to retaliate in the way you did."
He was fined $750 on both charges.
Stone, who said he had been a customer at the petrol station for 18 years, later told Hawke's Bay Today the injury was worse than anything he'd received during his year-long service in Vietnam.
"But I want it noted I acted not as a Vietnam War vet, but as myself," Stone said.
"His [Ryan's] was a shocking act as far as I'm concerned. I reacted simply because firstly he was rude to me, then he did something beyond belief. I just don't think a young fella should get away with that sort of thing. It was gutless, and I told him so after he punched me."
He said he considered the two sentences unfair.
"He's only got community work whereas I'm paying $1500 and face more surgery, including an eye-graft.
"That's not right."
J Young Motors Ltd owner, Dave Schofield, refused to comment. Asked if Ryan was still employed with the business he said: "What do you think?"
