Johnstone, who found Rio to be an incredible experience, has moulded mount Balmoral Sensation's horse shoe and two nails into gold at a jeweller's outlet.
"It's the best sporting moment of my life so far."
This week his horses, including young In Disguise at two-star level, are stepping up.
He considers his main horse, Quainton Labyrinth, earmarked for the marquee Olympic Cup on Sunday, talented but its experience may come under the microscope.
The Dunedin-born rider, who has etched his name on the silverware of the Hoy Show, Kihikihi, Puhinui, national three-day eventing title, was a member of the bronze-medal team to the World Equestrian Games in Kentucky 2010.
He was the highest-placed Kiwi Olympian in Rio, coming in sixth position, and his top mount there was Balmoral Sensation.
Johnstone won the Land Rover three-star eventing crown here last year on Balmoral Sensation but he won't be defending it or starting in the class.
"Horse of the Year is the biggest show domestically in the year so it's a major goal in its own right, but also helps you to prepare for international competition," he says, adding winning a title is always nice.
He is mindful of the rain forecast from tonight but, while he prefers sunshine, he believes it's unlikely to affect him and his horses.
"Some find it an advantage while others may see it as a disadvantage."
Jeff McVean, Vaughn Jefferis and Vanessa Way are among those who have helped carve his career.
Johnstone's sister, Grace, 27, encouraged him to ride as a youngster one day and it was instant love for the sport for him.
"I guess I was good at it. It's quite nice to be good at things," he says with a laugh.
He had played other sport with varying degrees of success before that.
Did Grace carry on with equestrian?
Johnstone says his sister was an accomplished rider but gave it up after leaving school.