Carissa McGregor (Christchurch) and Chatterton had some big air over most of the jumps but came unstuck at a chequered upright leaving them with four faults in 47.44.
Kim Best (Otane) and Giggles looked on track for a good time until that same chequered fence, which put them off stride for the next jump leaving them on eight faults in 47.86.
Last to go was McVean and the lovely grey Dunstan Zasjany D who left everything up in 47.08 to claim fourth spot.
"I thought if I tried to go flat out I would do something stupid,'' said McVean, who previously won the title in 2007 with Dunstan Hairy McClairy and 2010 with Dunstan Daffodil.
Zimorena was a late starter in the world of showjumping, having been broken in as a five-year-old on her arrival in New Zealand.
"She's good if I can keep her rolling on but she is green at this level,'' she said.
The eight-year-olds have finished the main part of their show, leaving the "big'' stuff up to Daffodil, who will start in the $50,000 Silver Fern Stakes and the $200,000 Olympic Cup.
The Horse of the Year Show continues until Sunday when the Olympic Cup will be presented to the showjumper of the year.
Australia and New Zealand go head to head on Friday night in the second of a two-test transtasman series.
RESULTS
Showjumping, lady rider of the year: Katie McVean (Mystery Creek) Dunstan Zimorena G 1, Rosie Commons (Auckland) Glenara Chandon 2, Evie Buller (Australia) Yalambi's Landor 3, Katie McVean (Mystery Creek) Dunstan Zasjany D 4, Claire Wilson (Waipukurau) Tipsey 5, Carissa McGregor (Christchurch) Chatterton 6.
Horse 1.4m: Daniel Meech (Taupo/Hawke's Bay) Argento (owned by Robert and Angi Bruce) 1, Maurice Beatson (Dannevirke) Conyers 2, Katie McVean (Mystery Creek) Dunstan Springfield 3, Katie McVean (Mystery Creek) Dunstan Delilah 4, Phillip Steiner (Tauranga) On The Point 5, Claudia Hay (Mosgiel) Euro Sport Heartbreaker 6
Showing, Gee Whizz Memorial: Dorothy Lennard (Te Aroha) Glen Tullock 1, Allie Harper (Winton) Dream Girl MH 2, Susan Reynolds (Hastings) Mancini 3, Alannah Flannagan (Auckland) Black Jack 4, Anne Hjorth (Cambridge) Acquisition 5, Anne Watts (Papakura) BJK Cashmir 6.
Lead rein rider of the year: Lucia Beckett (Albany) Gunda Gai Nadrill 1, Ryan Teece (Nelson) Merivale Park Serenade Rose 2, Brooke Cadwallader (Auckland) Nanteos Autumn Guardsman 3, Maia Amos (Ashhurst) Sherwoods Royal Emblem 4, Trey Cadwallader (Auckland) Cedar Lakes Angus 5, Kasey McKenna (Papakura) Lakevale Puss n Boots 6.
First-ridden rider of the year: Paris Masterson (Papakura) Leeara Park First Edition 1, Lily Webber (Franklin District) Jubilee Final Impression 2, Meg Fleming (Christchurch) Silver Shadow 3, Alyssa Fox (Carterton) Fortification `Scuse Me 4, Zoe Booth (Pukekohe) Lakevale Puss n Boots 5, Nikita Lamb (Carterton) Amerberley Falkor 6.
First-ridden pony of the year: Paris Masterson (Papakura) Leeara Park First Edition 1, Natasha Forsythe (Paeroa) Greenlee Aristocrat 2, Molly Pike (Cambridge) Uptown Isaac 3, Alyssa Fox (Carterton) Fortification `Scuse Me 4, Nicole Weatherley (Matamata) Windvale Classic Time 5, Lily Webber (Franklin) Jubilee Final Impression 6.