The derby course is a test of skill and endurance not often asked of show jumping horses.
To prepare for the event, Kewish is training with her 10-year-old Selle Francias-thoroughbred cross mare, Spring Fling, at venues around the wider Cambridge area.
"We are practicing jumping drop fences, ditches, going up banks and making sure she is confident on different kinds of terrain," said Kewish.
Kewish has been attending Equidays since its inception six years ago, mostly as a spectator. Last year she rode at Equidays in the FEI World Challenge qualifier (1.10-1.20m) and was placed ninth.
Equidays competition manager Kerry Willetts said they have been inundated with entries in the Wade Equine Derby. "We've intended to do it for a few years but we were worried it didn't have the appeal of the bigger classes," said Willetts.
"However it has proven to be very successful and the class is almost full."
Willetts said competitors in the class ranged from younger children to adults. Among them are "up-and-coming riders who will be our future stars".
Willetts has been involved with Equidays since it started in 2010. "The competitions have grown a lot. Each year we've added another new competition," he said.
Also new this year is the invitational Horseware Ireland Double Slalom, part of the Friday night show jumping spectacular.
An exciting event for spectators, at the Double Slalom two horse-and-rider combinations go head-to-head, racing each other on identical courses to beat each other and the clock.
Returning again this year, after much success in 2015 are the FEI World Jumping Challenge, the JLT Bloodstock Pony Six Bar, the Pony Grand Prix, the GH Mumm Champagne Horse Grand Prix, the Hyundai Premier Derby, Honda Pony Derby and the Isuzu D-Max Show jumping and Cross Country Challenge - Accumulator. The prize pool exceeds $60,000.
A key focus of Equidays is education and, alongside the competitions and entertainment, more than 40 hours of educational clinics and seminars will run daily led by top equine experts. Equidays 2016 also includes two spectacular night shows and more than 200 exhibitors.
* See equidays.co.nz or visit equestrianentries.co.nz for the full competition schedule.