The accident occurred at the 15th fence, an upright built from telegraph poles which had been jumped safely 200 times by previous competitors.
Having refused there, Smart's horse somersaulted over the fence and then landed on top of her. She was taken to Lincoln City Hospital, but was dead on arrival. The event, which was abandoned after Smart's accident, was the fifth equestrian fixture to be marred by the death of a rider this year.
The other four riders - Richard Adams, Malcolm Munro-Kerr, Mark Holliday and Vanessa Weaver - died while competing in horse trials. This latest death is the first to have occurred at a Pony Club fixture since 1984.
Another rider, 15-year-old Zoe Nesbitt, died in a schooling accident when riding at her home this spring. She had been training for a junior event when her horse fell after jumping over a table fence.