Rio Olympic rider Clarke Johnstone has scored the biggest win of his career today by taking the Australian international three-day event title in Adelaide.

Johnstone won the only Southern Hemisphere four star event with a score of 54.7 penalty points aboard Balmoral Sensation.

The pair produced a faultless showjumping round and had to, to pip Australian Megan Jones and Kirby Park Impress 3.7 penalty points back.

Johnstone, from Matangi in the Waikato, is the third New Zealander to win the title, after Nick Larkin on Red in 1997 and Matthew Grayling on Revo in 2001.

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Johnstone was New Zealand's best-performed rider in Rio, finishing sixth.

Adelaide is one of only six four-star class events in the eventing world. Johnstone scored 45.9 points in the dressage and had 8.8 time penalties on the cross country round.

Balmoral Sensation nudged a couple of early rails, but soon found his groove and Johnstone rode a brilliant clear to take the crown.

"I feel so very lucky to be here," he said.

''(To win a four star title) is something I have strived for my whole career since I started riding, and to finally be here is the best feeling."

Donna Edwards-Smith (Te Kauwhata) and DSE Mr Hokey Pokey were the next best of the Kiwis in seventh place on 73.9, after adding four faults in today's showjumping.

Ginny Thompson (Kaukapakapa) and Star Nouveau were eighth on 82.6, Andy Daines (Kumeu) 10th on Spring Panorama and Edwards-Smith 13th with DSE Tangolooma.

There was plenty of carnage in the showjumping round with just three combinations coming home clear and inside the required time.