The win cannot be overstated as the Australian International is one of six events throughout the year rated on the top circuit, alongside events such as the Badminton and Burghley Horse Trials. They are rated as ‘‘four-star’’ events.
It was no wonder Johnstone was rapt with the victory.
Johnstone, who was brought up on a property near Outram, started out well in the dressage, scoring 45.9 points to take the lead.
He then backed that up with just 8.8 time penalty points in the cross-country.
The pressure was then on in the final showjumping round.
Riding just in front of him was Megan Jones, of Australia, on Kirby Park Impress, who went through with a clear round, putting the pressure on Johnstone.
One rail would have cost him victory but apart from a couple of nudges early on, Balmoral Sensation went clear and with it Johnstone won the title.
Johnstone, who finished sixth at the Rio de Janeiro Olympics eventing last year, was elated.
"I feel so very lucky to be here. It is something I have strived for my whole life since I started riding — to win a four and to be finally here is the best feeling ever," he said.
He paid tribute to his parents and grandmother, who own Balmoral Sensation, his groom, Lydia Beales, and his team back at home.
Johnstone (30) also praised event organiser Gillian Rolton, who died on Saturday of cancer.
Rolton, a former Australian equestrian, won two Olympic gold medals and was the director of the Adelaide event.
"This event isn’t about me; it is about Gill. She was an amazing woman, who made the event into this, and we will all miss her very much.
"My heart goes out to Gill’s family and close friends. This is my favourite event in the whole world. There are no other events like this. I love coming here and I look forward to doing so again in the future."
Of the 20 original starters at the event, five were eliminated, one retired during the cross-country and one withdrew at the second inspection.
Johnstone was just the third New Zealander to win the event, behind Nick Larkin and Red in 1997, when it was a lower-rated event, and Matthew Grayling aboard Revo in 2001, after the event had risen in stature.
