He has entered Balmoral Sensation and Quainton Labyrinth in the Norwood Gold Cup, which will be staged at the Land-Rover Premier Arena from 1.30pm-3.45pm.
"The 1.45m fence is a lot higher than the eventing one so, hopefully, they'll be up to the job," he said of the mounts.
Ten days after competing here, Johnstone will jet off to England with 11-year-old gray gelding Balmoral Sensation to compete in Europe, including the Badminton Horse Trails.
"If everything goes well I should make the New Zealand team," said Johnstone who will be up against the class of Sir Mark Todd, Blyth Tait, Jock Paget, and Tim and Jonelle Price.
He is under no illusions about the enormity of the task that awaits him as only four from the six will go to Rio although not all of them will be at Badminton. The four-star Kentucky event in the States will beckon a few.
"They are all abroad and I'm the only one here so that's why I'm going to England to compete directly against them."
Johnstone said two of the four berths were "locked" while the others would jostle for the remaining two places.
He makes no secret of the fact that he didn't find much traction in his three-year stint in England despite some success there.
"The winters there are bloody terrible. I had some very good horses and they got injured," he said, emphasising he made a lifestyle choice to live in New Zealand.
"I chose to live here because it's also a good place to produce horses."
Past performances, he felt, would be a factor to go to Rio but it also boiled down to a matter of perception in selection in a "subjective sport".
But he is confident in his ability despite the heartache of the 2010 London Olympics when his mount, Orient Express, picked up an injury three weeks before the event.
"Coerced into riding" as a 12-year-old near Mosgiel in Otago, Johnstone gravitated towards equestrian after watching younger sister Grace doing it.
Balmoral Sensation, who can be "quiet or hyperactive" in their two-year rapport, has never let him down.
"I have to keep him calm and and he's very committed," said the rider who was the NZ Young Rider Champion in 2006 in Christchurch.
He will ride 8-year-old Bay gelding Quainton Labyrinth in the Premier Stakes on Saturday.
The Land Rover CIC3* eventing is lining up to be a cracker with a 17-strong class.
Tomorrow's events:
Land-Rover Premier Arena:
* Class 8, 10.30am-12.30pm: AHD Ltd 6-year-old Horse of the Year Championship, part of Mitavite 6YO National Series.
* Class 9, 1.30pm-3.45pm: Norwood Gold Cup 1.45m.
NRM Arena:
* Class 10, 9am-11.45am: Donut Express Horse 1.30m.
* Class 11, 12.15pm-1.45pm: GJ Gardner Pro Am Rider of the Year Championship, 1.20m-1.30m, part of the Let's Bale National Pro-Amateur Series.
* Class 12, 2.15pm-5pm: Pony 1.20m.
* Class 13 (a): Saba Sam Shield.
Fiber Fresh Arena two:
* Class 14 8am-1pm: Pro-Dosa BOOST Horse 1.20m.
* Class 15, 1.15pm-5.15pm: Pony 1.10m.
Arena three:
* Class 16, 8am-12.45pm: Pony 1.05m.
* Class 17, 1pm-7pm: Horse 1.10m.
Norsewood Cup:
Entries for the event starting at 1.30pm at the Premier Land-Rover Arena tomorrow (No/rider/mount):
1997: Clinton Beresford (Aust, Emmaville Jitterbug).
1543: Sally Spicer (Bella Vi).
1420: Kimberleigh McCabe (Kace).
1522: Rachel Malcolm (Equador MVNZ).
1662: Geordie Bull (Forest Hill)/
1272: Olivia Robertson (Ngahiwi Cisco).
1180: Katie Laurie (Dunstan On the Point Sandy).
1425: Georgia Massie (Larinium LS).
1204: Robert Wellwood (Cavallino).
1659: Oliver Edgecombe (Ultra Blue NZPH).
1396: Amanda Wilson (Showtym Cassanova).
1572: Clarke Johnstone (Balmoral Sensation).
1044: Catherine Cameron (WTF Cabello).
1096: Melody Matheson (Landenfeld).
1452: Katie Laurie (Dunstan Springfield).
1371: Joshua Barker (Eros K).
1291: Mathew Dickey (Quango).
1635: Maurice Beatson (Mandalay Cove).
1018: Nakeysha Lammers (All Expenses of Renton).
1601: Simon Wilson (McMillans Midway Smooth Dude).
1125: Laura Inkster (Strathcarron Zaurak).
1248: Samantha Morrison (Biarritz).
1548: Briar Burnett-Grant (Fiber Fresh Carlotta).
1712: Logan Massie (Mr Marmite).
1462: Emily Hayward-Morgan (A P Ninja).
1111: Carissa McCall (Esteban MVNZ).
1334: Nicki Booth (Collina).
1689: Kimberley Bird (Ngahiwi Summer).
1742: Clarke Johnstone (Quainton Labyrinth).
1442: Brooke Edgecombe (LT Holst Aunty Annette).
1118: Karen Niederer (Cape Palliser).
1653: Lucy Akers (Letitbe GNZ).
1016: Maurice Beatson (Conyers).
1233: Katie Laurie (Luego GNZ).