Johnstone, named yesterday in the New Zealand eventing team for Paris, has been grieving the death of his partner, Codey Jervis.
Mr Jervis, 32, died in Wānaka on New Year’s Eve. His body was recovered from Lake Wānaka, near Waterfall Creek, and his death was referred to the coroner.
"Having the goal of the Olympic Games, which we were both so invested in, has kept me going the past six months and I have poured my heart and soul into training and competing my lovely horses this year to make our goal a reality.
"I know he would be so proud."
The achievement was the culmination of years of "single-minded determination", training and moving to the other side of the world to be in the best possible position to challenge for medals in Paris, he said.
Johnstone has long been based in the UK but he grew up on a sheep and cattle farm at Outram.
He was introduced to the world of equestrian through his sister, and supported by parents Rob and Jean as he headed overseas after completing a bachelor of commerce at the University of Otago.
At 19, he was the youngest winner of an eventing world cup qualifier.
He was a member of the bronze medal-winning team at the 2010 world championships, and he won the FEI world cup eventing series in 2010-11.
Johnstone is lining up for his second Olympics, after debuting at Rio 2016, where his sixth individual placing was the best of the New Zealand riders.
He will compete aboard 14-year-old British sport horse Menlo Park, who appeared to be peaking at the perfect time for Paris, Johnstone said.
A seasoned New Zealand eventing team for Paris includes the former world No 1 and No 2 husband-and-wife combination of Tim and Jonelle Price, and Caroline Powell as travelling reserve.
Flying the flag in dressage is Olympic debutant Melissa Galloway.
Paris will be Jonelle Price’s fourth Games — she was a member of the bronze medal-winning team at London 2012, helped the team place fourth at Rio 2016, and was the leading Kiwi individual at Tokyo 2020, placing 11th.
Jonelle, who hails from Motueka but lives in the UK with Tim and their two children, will compete aboard her 12-year-old Dutch-bred mare Hiarado, with Grappa Nera named as her reserve horse.
"I am very delighted to be selected for my fourth Games," Jonelle Price said.
"The Olympic Games is the pinnacle of any sport and resonates so globally which makes it extra special."
World No 9 Tim Price made his Olympic debut at Rio 2016 and also rode at Tokyo 2020.
He has been named aboard the 15-year-old German-bred Hanoverian gelding Falco with the 12-year-old Selle Francais gelding Coup de Coeur Dudevin his reserve horse.
"I am very happy and excited to be involved in the Kiwi team again," he said.
"Both horses are in good form and healthy. Now it is about us binding together as a team and having that team result we are all so desperate for."
Dressage representative Galloway, of Marlborough, has a world ranking of 26, the highest ever for a Kiwi.
"I cannot explain what this means to me," she said.
"This has been my dream since I was 11 years old and now, 20 years later, to be actually realising it is unbelievable. I feel so honoured and grateful to everybody who has supported me on this whole journey for all those years."
Equestrian Sports New Zealand high performance general manager Jock Paget, himself a team bronze medal winner from the London 2012 Olympic Games, said the eventing selection had been particularly hard.
"We had lots of strong combinations to choose from," Paget said.
"It was the biggest selection headache I have witnessed in my time but we now have a really strong team who I think will be suitable for what we expect to go into in Paris."
