The 24-year-old world cup series winner will be teaching 30 riders skills which could improve their showjumping and cross-country abilities.
Taieri District Pony Club president Anne Beattie said when she became aware Johnstone was going to be in Dunedin she thought it would be a great opportunity for riders to improve their skills.
"Clarke belonged to the Taieri club and is passionate about the sport so we thought we should take the time to utilise his talents."
Riders from as young as 7 were coming from all over the lower South Island to take part, she said.
"It is really exciting that they will get the chance to learn from Clarke and he is looking forward to teaching them."
She was unsure how long Johnstone would be in Dunedin, but said he had to be back in the United Kingdom by January 10.
The clinic will focus on showjumping from 8am-6pm on Thursday, and on cross-country from 11am-9pm on Friday.
The show jumping will be held at Riverside, in Outram, while the cross-country will be held at the Wingatui racecourse.
Johnstone won the eventing world series in Paris in August. He hopes to compete in next year's London Olympics, but will not know if he has made the five-strong team until it is announced six to eight weeks before the Olympics start on July 27.
