"You couldn't hear yourself think, especially when he was last and then he started to go through," he told APNZ.
"He got down to the last 500m and he just shot through the whole lot of them."
Mr Sullivan said it was a very proud moment.
"I knew he'd do it. I've always said that he would."
Mr Sullivan said he was looking forward to seeing the gold medal.
"I don't know how big they are, but they look huge"
Mr Sullivan had not spoken to his grandson yet - although there was an excited message on his answerphone which he couldn't understand because people were "talking over one another".
It could be that the message was from Sullivan and his parents Mike and Elaine, who travelled to London to support their son to victory.
Picton Rowing Club member Colin Timms said there had been a great atmosphere in the "packed" Oxleys Hotel in Picton last night, where most of the club was gathered.
"It was a good crowd and we all yelled and cheered," he said.
"It was a bloody good effort - unbelievable."
Mr Timms said the crowd was quiet at first but there was "hooting and howling" as the boys surged ahead.
"They just rode through them, it was just great."
Mr Timms admitted he had "a bit of a headache" this morning after last night's celebrations - but said the mood would carry on in Picton today.
"There'll be some jubilant people around. It'll be great for Picton, for sure," he said.
The win was great news for the club, and Mr Timms hoped more people would want to join up now they had an Olympic hero in their midst.
"Picton has always had a pretty strong background in rowing, and hopefully young fellas will look at it and take it up."
