Stewart was assistant manager at the time, and he recalls his moment of disbelief, even though it was not uncommon to have notable visitors.
"It was late afternoon on my day in charge," he said. "Marineland had closed for the day and I was in the office completing the necessary paperwork.
"The receptionist was finishing up in the foyer.
"A phone call came in from the foyer and I was informed by the receptionist that there was someone outside who would like to come in to look around."
When he told them to say "Sorry, we were closed for the day," the receptionist said: "But it's Prince Edward!"
"Thinking someone was having me on, I went out to the foyer to be met by a couple of large, dark-suited men who informed me that Prince Edward was sitting outside in the car and would like to have a look around Marineland," Stewart said.
"He had just completed his stint as a housemaster at Wanganui Collegiate and was on his way back to Wellington before heading home," Stewart said.
"Perhaps his mum had told him about her 1970 Marineland visit, so I spent the next 20 minutes or so escorting the Prince and his bodyguards around the facility," he said.
The Prince was "extremely interested in the animals" and dolphins (Katie, Josephine, Kelly and Shona, he recalls) were more than happy, the Prince rolling up his sleeve and leaning from the platform to deliver their surprise extra feed for the day, and they got to "shake hands" with royalty.
The timing of the visit and the fact that the media had not been alerted, even to the fact royalty was in the area, suggested to Stewart that Prince Edward wanted "a little quiet time doing something he wanted to do without any fuss."
"I was certainly happy to oblige and was pretty chuffed with my first, and only, close encounter with royalty," he said.
He was almost as chuffed that so many years later it was worth a story.