"It wasn't until Nicole [White and Cricklewood Ace] finished her ride that I knew I had it," she said.
Robertson tries to keep things as simple as possible with just her longtime coach, John Cottle, and All Black father, Duncan and mum Jan Robertson, as well as Richard Sunderland.
She has been competing at the HOY Show for seven years and this was her fourth time in the young rider class.
"It's just a stepping stone to the future and it's fanastic to have won it, to be honest."
While she made second round in the class every year it was her first time in the yesterday's nail-biting jump off.
"It's just an mazing feeling. In some ways it still hasn't sunk in yet."
Robertson and Cisco will be in the elite field of JB Olympics class tomorrow in the premier event of the HOY Show from 2.15pm.
"It'll be my first time in the Olympic Cup and this was a fantastic preparation."
She was having a few celebratory drinks with friends last night after keeping the bragging rights in the South Island.
"I've had Rose and Nicole pushing me all season and they are fantastic riders so, hey, anyone of us could have won it on their day."
She labelled her mount "a one-in-a-million horse".
Claudia Hay, of Mosgiel, added to more Southern comfort in winning last night's $50,000 McMillan Equine Feeds Silver Fern Stakes on her black stallion, Euro Sport Centavos.
The Otago combination stole the limelight just-inducted Hall of Famer Katie Laurie, 2015 Lady Rider of the Year Lisa Cubitt, former Olympic Cup winner Maurice Beatson (Dannevirke) and Helen McNaught-McFarlane (Taupo).